Override user registration form
If you want to use the extended register form class, you need to put it in the place of the original form class in the user entity type:

mymodule.module

  /**
   * Implements hook_entity_type_alter().
   */
  function mymodule_entity_type_alter(array &amp;$entity_types) {
    $entity_types['user']-&gt;setFormClass('register', 'Drupal\mymodule\MyRegisterForm');
  }