There is not a form, per se. For that matter, if your bride to be and never worked on a "WP" Work Permit then she does not have to get written permission from MOM. If she has only been here on an "S" Pass, then permission is not required. It's ONLY for WP holders or former WP holders.
If she was on a WP BEFORE getting a "S" Pass, it would be a good idea to give MOM the benefit of the doubt and ask for permission anyway. It would be just a formality, but better safe than sorry.
sms