How do I make a lookup field display information and not the ID.info

How do I make a lookup field display information and not the ID.info

I have a "Contacts" field in a customer follow up form. This field looks up contacts in the contact form. Naturally if displays the ID.info. I've set a filter in the follow_up form to ID==site_ID. This should only display contacts that have the same ID as the ID entered in the Follow_Up form. The contact drop down selection box still displays the record ID.info. I've put this code in the "Contact" field On User Input.

FollowUpContact = Contacts [ID == input.Site_ID];
Site_Contact = FollowUpContact.Full_Name;

unfortunately the right side in the Follow_Up form is a string...

any thoughts?
Alan