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);
    });
");
?>