commit

app/docroot/modules/custom/mymodule/mymodule.links.task.yml

+mymodule.node.swapi:
+  route_name : mymdoule.test
+  title: Swapi
+  base_route: entity.node.canonical

app/docroot/modules/custom/mymodule/mymodule.module

@@ -69,7 +69,3 @@ function mymodule_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_
     $form["actions"]["reset"]["#access"] = true;
   }
 }
-
-function mymodule_preprocess_page() {
-
-}

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

@@ -4,3 +4,10 @@ mymodule.batch_form:
     _form: '\Drupal\mymodule\Form\BatchForm'
   requirements:
     _permission: 'administer site configuration'
+mymdoule.test:
+  path: '/node/{node}/swapi'
+  defaults:
+    _controller: '\Drupal\mymodule\Controller\SwapiLinkTask::exampleTabContent'
+    _title_callback: '\Drupal\Core\Entity\Controller\EntityController::title'
+  requirements:
+    _permission: 'administer content types'

app/docroot/modules/custom/mymodule/src/Controller/SwapiLinkTask.php

+<?php
+
+namespace Drupal\mymodule\Controller;
+
+use Drupal\Core\Controller\ControllerBase;
+use Drupal\node\NodeInterface;
+use Drupal\Core\Access\AccessResult;
+
+class SwapiLinkTask extends ControllerBase {
+  public function exampleTabContent() {
+    return ['#markup' => 'Content from swapi'];
+  }
+}