code: public function createUser()
      {
          if (false == $this->validateRegistrationOptin()) {
              //show error message on submit but not on page reload.
              if ($this->getRequestParameter('stoken')) {
                  \OxidEsales\Eshop\Core\Registry::get(\OxidEsales\Eshop\Core\UtilsView::class)->addErrorToDisplay('OEGDPROPTIN_CONFIRM_USER_REGISTRATION_OPTIN', false, true);
                  \OxidEsales\Eshop\Core\Registry::get(\OxidEsales\Eshop\Core\UtilsView::class)->addErrorToDisplay('OEGDPROPTIN_CONFIRM_USER_REGISTRATION_OPTIN', false, true, 'oegdproptin_userregistration');
              }
          } else {
              return parent::createUser();
          }
      }
docs: Create new user.
  
  @return mixed