Extend drupal 8 user register form

NewUserRegisterForm.php

<?php

namespace Drupal\my_module\Form;


use Drupal\Component\Datetime\TimeInterface;
use Drupal\Core\Entity\EntityFormBuilderInterface;
use Drupal\Core\Entity\EntityManagerInterface;
use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
use Drupal\Core\Extension\ModuleHandlerInterface;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Language\LanguageManagerInterface;
use Drupal\user\Entity\User;
use Drupal\user\RegisterForm;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Provides a user login form.
 */
class NewUserRegisterForm extends RegisterForm {

    public function __construct(EntityManagerInterface $entity_manager, LanguageManagerInterface $language_manager, EntityTypeBundleInfoInterface $entity_type_bundle_info = NULL, TimeInterface $time = NULL, ModuleHandlerInterface $moduleHandler) {
        $this->setEntity(new User([], 'user'));
        $this->setModuleHandler($moduleHandler);

        parent::__construct($entity_manager, $language_manager, $entity_type_bundle_info, $time);
    }

    /**
     * @inheritdoc
     */
    public static function create(ContainerInterface $container) {
        return new static(
            $container->get('entity.manager'),
            $container->get('language_manager'),
            $container->get('entity_type.bundle.info'),
            $container->get('datetime.time'),
            $container->get('module_handler')
        );
    }

    public function form(array $form, FormStateInterface $form_state) {
        $form = parent::form($form, $form_state);
        $form['test'] = [
            '#markup' => '<p>Test extended form</p>',
        ];
        return $form;
    }
}

RouteSubscriber.php

<?php

namespace Drupal\my_module\Routing;


use Drupal\Core\Routing\RouteSubscriberBase;
use Symfony\Component\Routing\RouteCollection;

/**
 * Listens to the dynamic route events.
 */
class RouteSubscriber extends RouteSubscriberBase {

    /**
     * {@inheritdoc}
     */
    protected function alterRoutes(RouteCollection $collection) {
        if ($route = $collection->get('user.register')) {
            $route->setDefault('_form', '\Drupal\my_module\Form\NewUserRegisterForm');
        }

    }
}

This is actually a bit of a hack. You shouldn't need to implement __constuct() and create() if you're just changing markup.

If all you want to do is extend the form but not override the route, implement hook_entity_type_alter in your .module file like so:

function your_module_entity_type_alter(array &$entity_types) {
  $entity_types['user']->setFormClass('register', '\Drupal\your_module\Form\NewRegisterForm');
}

And in your NewRegisterForm.php you don't need to implement __construct or create, as it inherits those from RegisterForm.php. In my case, I just wanted to change the submit button text, so I have:

class NewRegisterForm extends RegisterForm {

  protected function actions(array $form, FormStateInterface $form_state) {
    $element = parent::actions($form, $form_state);
    $element['submit']['#value'] = 'Submit';
    return $element;
  }

}

The class that is used to build the form is set on the User object in annotations in User.php, so you need to change it there.

I am getting

The website encountered an unexpected error. Please try again later.

Drupal\Core\Entity\EntityStorageException: SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'uuid' cannot be null: INSERT INTO {users} (uid, uuid, langcode) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2); Array
(
[:db_insert_placeholder_0] => 26
[:db_insert_placeholder_1] =>
[:db_insert_placeholder_2] => en
)

When I try submit the form? Any suggestions?

For those how have the same problem, this is the solution :
-> You need to create a new instance of user in the contructor as this below

  public function __construct(
    EntityRepositoryInterface $entity_repository,
    LanguageManagerInterface $language_manager,
    EntityTypeBundleInfoInterface $entityTypeBundleInfo = NULL,
    TimeInterface $time = NULL,
    ModuleHandlerInterface $moduleHandler
  ) {
    $user = User::create();
    $this->setEntity($user);
    $this->setModuleHandler($moduleHandler);

    parent::__construct($entity_repository, $language_manager, $entityTypeBundleInfo, $time);
  }

Hope it will help !

In case anyone else stumbles upon this: if errors such as missing fields occur, you may have forgotten to submit the parent form.