this is modal window task

app/docroot/modules/custom/formApi/formApi.module

+<?php
+
+/**
+ * Implements hook_theme().
+ */
+function formApi_theme($existing, $type, $theme, $path) {
+  return [
+    'form_theme' => [
+      'variables' => [],
+    ],
+  ];
+}
+/**
+ * Implements template_preprocess_HOOK().
+ * Обработчик переменных для первого примера.
+ */
+function template_preprocess_form_theme(&$variables) {
+  $myConfigPage = \Drupal\config_pages\Entity\ConfigPages::config('custom_config');
+  $variables['string_modal'] = $myConfigPage->get('field_modal_title')->value;
+  $current_user = \Drupal::currentUser();
+  $user = \Drupal\user\Entity\User::load($current_user->id());
+  $name  = $user->get('name')->value;
+  $user_last_name =  $user->field_last_name->value;
+  if ($name == NULL){
+
+    $variables['result'] = $variables['string_modal'].", гость!";
+  }
+  else {
+    $variables['result'] = $variables['string_modal']. ', ' . $user_last_name. ' ' .$name;
+  }
+}

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

@@ -14,7 +14,7 @@ use Drupal\Core\Ajax\RedirectCommand;
 use Drupal\Core\Ajax\RemoveCommand;
 use Drupal\Core\Form\FormBase;
 use Drupal\Core\Form\FormStateInterface;
-
+use Drupal\Core\Ajax\OpenModalDialogCommand;
 class FormApiTask extends FormBase {
   /**
    * {@inheritdoc}
@@ -189,7 +189,11 @@ class FormApiTask extends FormBase {
         ])
         ->execute();
 
-      $response->addCommand(new AlertCommand('Спасибо за заполнение, нажмите "OK" для продолжения'));
+      $content1['#attached']['library'][] = 'core/drupal.dialog.ajax';
+      $content1['#theme'] = 'form_theme';
+      $title = 'Сообщение';
+      $response->addCommand(new OpenModalDialogCommand($title, $content1, ['width' => '200', 'height' => '200']));
+      //$response->addCommand(new AlertCommand('Спасибо за заполнение, нажмите "OK" для продолжения'));
       $url = '/';
       $response->addCommand(new RedirectCommand($url));
 

app/docroot/modules/custom/formApi/templates/form-theme.html.twig

+<div style="color:blue">
+{{ result }}
+</div>

app/docroot/modules/custom/myservice/myservice.module

@@ -30,7 +30,7 @@ function myservice_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form
   if ($form_id == 'contact_message_contact_form_form') {
     $current_user = \Drupal::currentUser();
     $user = \Drupal\user\Entity\User::load($current_user->id());
-    $name  = $user->name->value;
+    $name  = $user->get('name')->value;
     $user_last_name =  $user->field_last_name->value;
     $form['field_last_name']['widget'][0]['value']['#default_value'] = $user_last_name;
     $form['field_name']['widget'][0]['value']['#default_value'] = $name;