How to make modal popup automatically when website is loaded

Question

        <div class="modal-dialog modal-dialog-centered">
          <div class="modal-content">

            <!-- Modal Header -->
            <div class="modal-header">
              <h4 class="modal-title">Modal Heading</h4>
              <button type="button" class="close" data-dismiss="modal">&times;</button>
            </div>

            <!-- Modal body -->
            <div class="modal-body">
              Modal body..
            </div>
           </div>
        </div>
    </div>

How to open popup when website is loaded when user visit my website they should get popup