Comments - Dropdown box selection?

I have altered the comments form to include a dropdown box. However, I cannot figure out the Kohana code to enable this to work.

Currently a field value is inputed into the database using, for example - $comment->guest_url = $form->add_comment->url->value;

And I have read that in Kohana, you use the following method - $this->request->post('dropdownlistNAME');

How would I integrate this into the current Comments coding to work?

Any help would be greatly appreciated, thanks!