degov-8.x-2.0/modules/degov_rich_text_format_settings/degov_rich_text_format_settings.module

<?php

use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_FORM_ID_alter().
 */
function degov_rich_text_format_settings_form_entity_embed_dialog_alter(&$form, FormStateInterface $form_state, $form_id) {
  // Add autocompletion to the link url field of the entity embed dialog. This
  // requires the patch added to this module's composer.json and the Linkit
  // module, which is a dependency of this module.
  if (isset($form['attributes']['data-entity-embed-display-settings']['link_url'])) {
    $form['attributes']['data-entity-embed-display-settings']['link_url']['#type'] = 'linkit';
    $form['attributes']['data-entity-embed-display-settings']['link_url']['#autocomplete_route_name'] = 'linkit.autocomplete';
    $form['attributes']['data-entity-embed-display-settings']['link_url']['#autocomplete_route_parameters'] = [
      'linkit_profile_id' => 'full_text',
    ];
  }
}