pop up populating another field in a form

Having difficulty with this.

Requirement:

I have a field called deliverable name. Pop up provides a list of possible deliverable names that COULD populate the deliverable name field. If the desired deliverable name is not in the pick list (coming from a look up table without any relationships), then the user can enter what he wants. How can i do this?

Requirement would be to provide a pick list button or icon next to the deliverable field input. When clicked, a pop up would appear. User would select value, and it would pre-populate field. If no value exists in pop up list, then user can enter anything (string). ANy direction is quite appreciated.

You could try flexbox[dot]codeplex[dot]com/ (sorry - still can’t include links) - it seems to do what you want.