如何验证amp-form中的选择字段不允许默认启动选项？

<form id="myForm"
  method="post"
  action-xhr="/submit"
  custom-validation-reporting="as-you-go"
  target="_top">

  <label for="mySelect">选择字段:</label>
  <select id="mySelect" name="mySelect" required>
    <option value="">请选择</option>
    <option value="option1">选项1</option>
    <option value="option2">选项2</option>
    <option value="option3">选项3</option>
  </select>

  <input type="submit" value="提交">
</form>

<script>
  document.getElementById('myForm').addEventListener('submit', function(event) {
    var selectField = document.getElementById('mySelect');
    if (selectField.checkValidity()) {
      // 没有默认启动选项
      console.log('选择字段没有默认启动选项');
    } else {
      // 存在默认启动选项
      console.log('选择字段有默认启动选项');
    }
    event.preventDefault();
  });
</script>