如果没有设置会话变量，如何在窗口加载时打开bootstrap4弹出窗口？

<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@4.6.0/css/bootstrap.min.css">
<script src="https://cdn.jsdelivr.net/npm/bootstrap@4.6.0/js/bootstrap.min.js"></script>

<button type="button" class="btn btn-primary" data-toggle="modal" data-target="#myModal">
  打开弹出窗口
</button>

<div id="myModal" class="modal fade">
  <div class="modal-dialog">
    <div class="modal-content">
      <!-- 弹出窗口的内容 -->
      <div class="modal-header">
        <h5 class="modal-title">弹出窗口标题</h5>
        <button type="button" class="close" data-dismiss="modal">&times;</button>
      </div>
      <div class="modal-body">
        <p>这里是弹出窗口的内容。</p>
      </div>
      <div class="modal-footer">
        <button type="button" class="btn btn-secondary" data-dismiss="modal">关闭</button>
        <button type="button" class="btn btn-primary">保存</button>
      </div>
    </div>
  </div>
</div>

<script>
  $(document).ready(function(){
    $('#myModal').modal('show');
  });
</script>