Well...that would be the one that's actually holding the pen, right?
OH COME ON...That's hilarious! ;D
Unless your boyfriend is a co-signer (which he can't be), you don't need to submit that form. Some have still opted to do so, in the hopes that it helps with their evidence of relationship, but as you will see on the document checklist, it's not required.
If you do fill it out, you would both need to sign it and it wouldn't matter which one of you filled it out.
Good luck!
*edit* Oops...didn't see that canuck_in_uk beat me to this one.