<button id="openModal">打开表单</button>

#myModal {
  display: none; /* 默认隐藏弹出窗口 */
  position: fixed;
  z-index: 1;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
  background-color: rgba(0, 0, 0, 0.4); /* 半透明黑色背景 */
}

.modal-content {
  background-color: #fefefe;
  margin: 15% auto;
  padding: 20px;
  border: 1px solid #888;
  width: 80%;
}

// 获取按钮和弹出窗口元素
var btn = document.getElementById("openModal");
var modal = document.getElementById("myModal");

// 点击按钮时显示弹出窗口
btn.onclick = function() {
  modal.style.display = "block";
}

// 点击弹出窗口外部或关闭按钮时隐藏弹出窗口
window.onclick = function(event) {
  if (event.target == modal) {
    modal.style.display = "none";
  }
}

<div id="myModal" class="modal">
  <div class="modal-content">
    <span class="close">&times;</span>
    <form>
      <!-- 在此添加表单元素 -->
      <label for="name">姓名:</label>
      <input type="text" id="name" name="name" required>
      <br>
      <label for="email">邮箱:</label>
      <input type="email" id="email" name="email" required>
      <br>
      <input type="submit" value="提交">
    </form>
  </div>
</div>