Add a role on user create

In some cases, you may want to add a role when a new user is created regardless of how the user is created (UI, GraphQL, Rest API, or something else). The hook_ENTITY_TYPE_presave() hook is perfect for this.

use Drupal\user\UserInterface;

function MY_MODULE_user_presave(UserInterface $user) {
  if ($user->isNew() && !$user->hasRole('frontend_app')) {
    $user->addRole('frontend_app');
  }
}