<form>
  <label for="field">特定字段:</label>
  <input type="text" id="field" name="field">

  <div id="additional-section" style="display: none;">
    <!-- 添加的部分 -->
    <label for="additional-field">额外字段:</label>
    <input type="text" id="additional-field" name="additional-field">
  </div>
</form>

<script>
  var fieldInput = document.getElementById('field');
  var additionalSection = document.getElementById('additional-section');

  fieldInput.addEventListener('input', function() {
    if (fieldInput.value === '某项内容') {
      additionalSection.style.display = 'block';
    } else {
      additionalSection.style.display = 'none';
    }
  });
</script>