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// 1. Create a custom module.
// ...

// 2. Implement Hook Form Alter.
function mymodule_form_alter(&$form, \Drupal\Core\Form\FormStateInterface $form_state, $form_id) {
  if ($form_id == 'node_CONTENT_TYPE_edit_form') {
    // Add your predefined select field.
    $form['predefined_select'] = array(
      '#type' => 'select',
      '#title' => t('Predefined Select Field'),
      // Add your options here.
      '#ajax' => array(
        'callback' => 'mymodule_update_second_select',
        'event' => 'change',
        'wrapper' => 'second-select-wrapper',
      ),
    );

    // Add a wrapper for the second select field.
    $form['second_select_wrapper'] = array(
      '#prefix' => '<div id="second-select-wrapper">',
      '#suffix' => '</div>',
    );
  }
}

// 3. Implement JavaScript (mymodule.libraries.yml).
custom-scripts:
  js:
    js/custom.js: {}

// 4. Implement JavaScript (custom.js).
(function ($) {
  Drupal.behaviors.mymoduleCustomSelect = {
    attach: function (context, settings) {
      $('#edit-predefined-select', context).once('mymodule-predefined-select').change(function() {
        // Trigger an AJAX call.
        Drupal.ajax({url: '/mymodule/ajax/update-second-select'}).execute();
      });
    }
  };
})(jQuery);

// 5. Implement AJAX Callback (mymodule.routing.yml).
mymodule.ajax_update_second_select:
  path: '/mymodule/ajax/update-second-select'
  defaults:
    _controller: '\Drupal\mymodule\Controller\MyModuleController::updateSecondSelect'
    _title: 'Update Second Select'

// 6. Implement Controller for AJAX Callback (MyModuleController.php).
namespace Drupal\mymodule\Controller;

use Drupal\Core\Controller\ControllerBase;
use Symfony\Component\HttpFoundation\JsonResponse;

class MyModuleController extends ControllerBase {
  public function updateSecondSelect() {
    // Implement your logic to update the options of the second select field.
    // Return the updated HTML for the second select field.
    return new JsonResponse(array(
      'data' => render($second_select_element),
    ));
  }
}