在下拉框中选择项目时更改文本框值

document.getElementById("my-textbox").value = "New Value";

<input type="text" value="New Value" />

input[type="text"] {
  /* styles here */
}

<div class="form-group">
  <label for="my-textbox">Textbox Label</label>
  <input type="text" class="form-control" id="my-textbox" value="New Value" />
</div>

<textarea id="my-textbox" name="my-textbox" value="New Value" />