this is form api task

app/docroot/modules/custom/formApi/formApi.info.yml

+name: Form api
+type: module
+description: Module for implementing custom form.
+core_version_requirement: ^8.8 || ^9
+package: Custom

app/docroot/modules/custom/formApi/formApi.install

+<?php
+use Drupal\Core\Database\Database;
+
+/**
+ * Implements hook_schema().
+ */
+function formApi_schema(){
+    $schema['students'] = array(
+        'description' => 'The table for storing the students data.',
+        'fields' => array(
+            'id' => array(
+                'description' => 'The primary identifier for row',
+                'type' => 'serial',
+                'not null' => TRUE,
+                'unsigned' => TRUE,
+            ),
+            'name' => array(
+                'description' => 'Name',
+                'type' => 'varchar',
+                'length' => 255,
+                'not null' => TRUE,
+                'default' => '',
+            ),
+           'email' => array(
+                'description' => 'Email',
+                'type' => 'varchar',
+                'length' => 255,
+                'not null' => TRUE,
+                'default' => '',
+            ),
+            'site_address' => array(
+                            'description' => 'Site address',
+                            'type' => 'varchar',
+                            'length' => 255,
+                            'not null' => TRUE,
+                            'default' => '',
+                        ),
+
+        ),
+        'primary key' => array('id'),
+    );
+    return $schema;
+}

app/docroot/modules/custom/formApi/formApi.routing.yml

+formApi.form:
+  path: '/formApi'
+  defaults:
+    _form: '\Drupal\formApi\Form\FormApiTask'
+    _title: 'Form API'
+  requirements:
+    _permission: 'access content'

app/docroot/modules/custom/formApi/src/Form/FormApiTask.php

+<?php
+/**
+ * @file
+ * Contains \Drupal\formApi\Form\FormApiTask.
+ */
+namespace Drupal\formApi\Form;
+
+use Drupal\Core\Ajax\AddCssCommand;
+use Drupal\Core\Ajax\AfterCommand;
+use Drupal\Core\Ajax\AjaxResponse;
+use Drupal\Core\Ajax\AlertCommand;
+use Drupal\Core\Ajax\HtmlCommand;
+use Drupal\Core\Ajax\RedirectCommand;
+use Drupal\Core\Ajax\RemoveCommand;
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+
+class FormApiTask extends FormBase {
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'student_registration_form';
+  }
+
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $form['name'] = array(
+      '#type' => 'textfield',
+      '#title' => t('Name:'),
+    );
+    $form['site_address'] = array (
+      '#type' => 'textfield',
+      '#title' => ('Site address:'),
+      );
+    $form['system_messages'] = [
+      '#markup' => '<div id="form-system-messages"></div>',
+      '#weight' => -100,
+    ];
+    $form['actions']['#type'] = 'actions';
+    $form['actions']['submit'] = [
+      '#type' => 'submit',
+      '#name' => 'submit',
+      '#value' => 'Submit this form',
+      '#ajax' => [
+        'callback' => '::ajaxSubmitCallback',
+        'event' => 'click',
+        'progress' => [
+          'type' => 'throbber',
+        ],
+      ],
+    ];
+
+    $form['email'] = [
+      '#title' => 'Email',
+      '#type' => 'email',
+      '#required' => TRUE,
+    ];
+
+    return $form;
+  }
+
+  public function validateForm(array &$form, FormStateInterface $form_state)
+  {
+  }
+
+  public function ajaxSubmitCallback(array &$form, FormStateInterface $form_state){
+
+    $response = new AjaxResponse();
+    //обнуление стилей при повторном сабмите - необходимо удалить стиль надписи под полем, чтобы их не было 1000
+    $selector = '.domen-error';
+    $response->addCommand(new RemoveCommand($selector));
+    $selector = '.email-error';
+    $response->addCommand(new RemoveCommand($selector));
+    $selector = '.name-error';
+    $response->addCommand(new RemoveCommand($selector));
+
+    //ВАЛИДАЦИЯ адреса сайта
+    $reg = '/(https?:\/\/)?(www.)?([\da-z\.-]+)\.([a-z\.]{2,6})/ui';
+    if(preg_match($reg, $form_state->getValue('site_address'),$array_of_site_address))
+    {
+      // Если пользователь повторно ввел все нормально в это поле, то при сабмите сбросим красную рамку
+      if (end($array_of_site_address) === 'ru' || end($array_of_site_address) === 'рф'){
+        $style = '<style>#edit-site-address{border:1px solid grey !important;}</style>';
+        $response->addCommand(new AddCssCommand($style));
+       // при успешной валидации убираем из результата http или https или www
+
+        $result = preg_replace('/((https?:\/\/)||(www\.))/ui', '', $form_state->getValue('site_address'));
+        $valid_site_address = true;
+
+      }
+      else {
+        //ошибка домена
+        $style = '<style>#edit-site-address{border:3px solid red}</style>';
+        $response->addCommand(new AddCssCommand($style));
+        $selector = '#edit-site-address';
+        $content = '<p class = "domen-error">Вы ввели неправильную доменную зону, необходимая зона: "рф" или "ru"</p>';
+        $response->addCommand(new AfterCommand($selector, $content));
+        $style = '<style>.domen-error{color: red !important;}</style>';
+        $response->addCommand(new AddCssCommand($style));
+      }
+    }
+    else {
+      //ошибка ввода самого адреса
+      $style = '<style>#edit-site-address{border:3px solid red}</style>';
+      $response->addCommand(new AddCssCommand($style));
+      $selector = '#edit-site-address';
+      if ($form_state->getValue('site_address') == NULL){
+        $content = '<p class = "domen-error">Необходимо ввести адрес сайта</p>';
+      }
+      else {
+        $content = '<p class = "domen-error">Вы ввели адрес неправильно, пример адреса "www.example.ru"</p>';
+      }
+
+      $response->addCommand(new AfterCommand($selector, $content));
+      $style = '<style>.domen-error{color: red !important;}</style>';
+      $response->addCommand(new AddCssCommand($style));
+    }
+
+     // ВАЛИДАЦИЯ email
+     if (filter_var($form_state->getValue('email'), FILTER_VALIDATE_EMAIL))
+     {
+       $style = '<style>#edit-email{border:1px solid grey !important;}</style>';
+       $response->addCommand(new AddCssCommand($style));
+       $valid_email = true;
+     }
+     else
+     {
+       $style = '<style>#edit-email{border:3px solid red}</style>';
+       $response->addCommand(new AddCssCommand($style));
+       $selector = '#edit-email';
+
+       if ($form_state->getValue('email') == NULL){
+         $content = '<p class = "email-error">Необходимо ввести email</p>';
+       }
+       else {
+         $content = '<p class = "email-error">Вы ввели email неправильно, пример адреса "example@mail.ru"</p>';
+       }
+
+       $response->addCommand(new AfterCommand($selector, $content));
+       $style = '<style>.email-error{color: red !important;}</style>';
+       $response->addCommand(new AddCssCommand($style));
+     }
+
+    //Валидация name
+    if ($form_state->getValue('name') == NULL){
+
+      $style = '<style>#edit-name{border:3px solid red}</style>';
+      $response->addCommand(new AddCssCommand($style));
+      $selector = '#edit-name';
+      $content = '<p class = "name-error">Необходимо ввести имя</p>';
+      $response->addCommand(new AfterCommand($selector, $content));
+      $style = '<style>.name-error{color: red !important;}</style>';
+      $response->addCommand(new AddCssCommand($style));
+
+    }
+    else {
+      $valid_name = true;
+      $style = '<style>#edit-name{border:1px solid grey !important;}</style>';
+      $response->addCommand(new AddCssCommand($style));
+    }
+
+
+
+
+    //после валидации обращаюсь к БД и заполняю поля у таблицы
+
+    if ($valid_email && $valid_name && $valid_site_address){
+
+      $connection = \Drupal::service('database');
+      $result = $connection->insert('students')
+        ->fields([
+          'name' => $form_state->getValue('name'),
+          'email' => $form_state->getValue('email'),
+          'site_address' => $result,
+        ])
+        ->execute();
+
+      $response->addCommand(new AlertCommand('Спасибо за заполнение, нажмите "OK" для продолжения'));
+      $url = '/';
+      $response->addCommand(new RedirectCommand($url));
+
+    }
+
+
+    return $response;
+
+  }
+
+
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+
+  }
+
+}