<div id="dropdown">
  <select id="options">
    <option value="option1">Option 1</option>
    <option value="option2">Option 2</option>
    <option value="option3">Option 3</option>
  </select>
</div>

<input type="text" id="textbox" readonly>

// 获取下拉选项和文本框的元素
var dropdown = document.getElementById("dropdown");
var options = document.getElementById("options");
var textbox = document.getElementById("textbox");

// 添加点击事件监听器
dropdown.addEventListener("click", function() {
  // 将选中的下拉选项值赋给文本框
  textbox.value = options.value;
});