html button

<a class="btn btn-primary js_openWindowModal" data-toggle="modal" data-target="#windowModal" href="/form/callme">Callme</a>

bootstrap 4

<!-- Modal -->
<div class="modal fade" id="windowModal" tabindex="-1" aria-labelledby="windowModalLabel" aria-hidden="true">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-body"><img src="preload.gif"></div>
    </div>
  </div>
</div>

Yii2 js

$this->registerJs("
    $(document).on('click', '.js_openWindowModal', function () {
            $('#windowModal').modal('show').find('.modal-body').load($(this).href);
    });
");
?>

Yii2 Controller

public function actionCallme() {

    $model = new CallForm();

    if ($model->load(Yii::$app->request->post()) && $model->validate()) {

        try {
                $this->service->create($model);
                if (Yii::$app->request->isAjax) {
                    return Json::encode(array('status' => 'success', 'type' => 'success', 'message' => 'Created successfully.'));
                }
                else {
                    Yii::$app->session->setFlash('success', 'Данные изменены');
                    $url = Url::previous();
                    return $this->redirect($url ?: Url::to(['index']));
                }
                
        } catch (Exception $ex) {
            Yii::$app->errorHandler->logException($e);
            if (Yii::$app->request->isAjax) {
                return Json::encode(array('status' => 'warning', 'type' => 'warning', 'message' => 'Can not created.'));
            } else {
                Yii::$app->session->setFlash('error', $e->getMessage());
            }
        }
    }
    if (Yii::$app->request->isAjax) {
        return $this->renderAjax('view', [
                    'model' => $model,
        ]);
    } else {
        return $this->render('view', [
                    'model' => $model,
        ]);
    }
}