How to insert form when theme uses an entry box

I am using a GraphPaperPress Photo-workshop theme. In the theme is a field called "Signup Page" and the box for entry has a sample entry of "http://www.example.com". I'd like to just enter the id of the form but not sure if this is correct. I realize you don't know the GPP theme, but in general can you point me in the right direction for themes with a signup page "box"?

My URL: http://mdavisphoto.us/workshops2