code: public function actionRegister()
      {
          if (!$this->module->enableRegistration) {
              throw new NotFoundHttpException;
          }
  
          $model = \Yii::createObject(RegistrationForm::className());
  
          $this->performAjaxValidation($model);
  
          if ($model->load(\Yii::$app->request->post()) && $model->register()) {
              return $this->render('/message', [
                  'title'  => \Yii::t('user', 'Your account has been created'),
                  'module' => $this->module,
              ]);
          }
  
          return $this->render('register', [
              'model'  => $model,
              'module' => $this->module,
          ]);
      }
docs: Displays the registration page.
  After successful registration if enableConfirmation is enabled shows info message otherwise redirects to home page.
  @return string
  @throws \yii\web\HttpException