Node.js: how to read user select choice and serve file contents in textarea upon submit

Node.js beginner here so bear with me. Maybe I'm a bit slow but I cannot figure out how to read a select box choice after a user submits it and then take some action based on what the choice is. Basically, I have something like this in the HTML

<select>
  <option value="volvo">Volvo</option>
  <option value="saab">Saab</option>
  <option value="mercedes">Mercedes</option>
  <option value="audi">Audi</option>
</select>
<input type="submit" value="Submit" />
<textarea id="textarea" rows="10" cols="100">
</textarea>

Now all I want to do is when a user chooses Volvo, and submits, a file (volvo.txt) is read from the server and the contents of that file are included in the textarea. Whether the user is redirected to some /submit sub-site and the same stuff is shows as on the start page is irrelevant. It seems like this should be an easy task but maybe I'm missing something obvious. Thanks for your help!

Edit: forgot to mention that I'm looking for a server-side solution that utilizes Node.js.