<?php

namespace Drupalbusiness_listingForm;

use DrupalCoreFormFormStateInterface;
use DrupaluserRegisterForm;

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

    /**
     * {@inheritdoc}
     */
    public function form(array $form, FormStateInterface $form_state) {

        $form = parent::form($form, $form_state);

        // Change the form here
        $form['test'] = [
          '#markup' => '<p>Test extended form</p>',
        ];
        return $form;
    }

// uncommenting submit results in error
//  public function submitForm(array &$form, FormStateInterface $form_state) {

//      get the created users uuid & perform additional database queries

//  }

/**
 * Implements hook_entity_build
 */
function business_listing_entity_type_build(array &$entity_types) {
    $entity_types['user']->setFormClass('business_listing_register', 'Drupalbusiness_listingFormNewUserRegisterForm');
}

public function register() {
    $build = [];

    $entity = Drupal::entityManager()
        ->getStorage('user')
        ->create(array());

    $formObject = Drupal::entityManager()
        ->getFormObject('user', 'business_listing_register')
        ->setEntity($entity);

    $build['form'] = Drupal::formBuilder()->getForm($formObject);
    return $build;
}