popup.php

<!-- Modal -->
<div class="modal fade" id="exampleModal" tabindex="-1" aria-labelledby="exampleModalLabel" aria-hidden="false">
    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-header">
                <h1 class="modal-title fs-5" id="exampleModalLabel">Pendafataran Berhasil!</h1>
                <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
            </div>
            <div class="modal-body"> Formulir anda berhasil terkirim. </div>
        </div>
    </div>
</div>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.6.4/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.4.1/js/bootstrap.min.js"></script>
<script>
  $(document).ready(function() {
      $("#exampleModal").modal('show');
  }); 
</script>