RegisterForm::form

public RegisterForm::form(array $form, FormStateInterface $form_state)

public function form(array $form, FormStateInterface $form_state) {
  $user = $this->currentUser();
  /** @var \Drupal\user\UserInterface $account */
  $account = $this->entity;
  $admin = $user->hasPermission('administer users');
  // Pass access information to the submit handler. Running an access check
  // inside the submit function interferes with form processing and breaks
  // hook_form_alter().
  $form['administer_users'] = array(
    '#type' => 'value',
    '#value' => $admin,
  );

  $form['#attached']['library'][] = 'core/drupal.form';

  // For non-admin users, populate the form fields using data from the
  // browser.
  if (!$admin) {
    $form['#attributes']['data-user-info-from-browser'] = TRUE;
  }

  // Because the user status has security implications, users are blocked by
  // default when created programmatically and need to be actively activated
  // if needed. When administrators create users from the user interface,
  // however, we assume that they should be created as activated by default.
  if ($admin) {
    $account->activate();
  }

  // Start with the default user account fields.
  $form = parent::form($form, $form_state, $account);

  return $form;
}