Simplenews block submission without reloading using Ajax - Drupal 9?

Simplenews block AJAX submission - Drupal 8 / 9

Simplenews module:
URL: https://www.drupal.org/project/simplenews

1. Enable the simplenews module
2. Change the form id (if you want) - Go to /admin/structure/block/manage/simplenewssubscription
3. Assign it to one region
4. Implement the custom code with your custom module.

That's it simple.