AFAIK you would assign an unused FN to that key, and assign to it a macro that presses shift and slash. I'm not aware of any other method, since there is no keyboard code that a keyboard could send that would indicate a single key producing the question mark. (It's important to understand this if you're using the question mark as a part of a some keyboard combo on the computer - it will be effectively, say, C-A-S-/ instead of C-A-?.)
Here's what's referenced for this on TMK FAQ:
http://deskthority.net/workshop-f7/tmk- ... ml#p195620