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');
        }

    }
}

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