html button

<a class="btn btn-primary" 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>