if I'm driving for employment I need a letter from my boss, right?
If you have an ignition interlock restriction your employer must provide you with signed documentation, on company letterhead, saying they have knowledge of the ignition interlock restriction.   The letter must also give you permission to operate the vehicle for business purposes without the device.
which form do I need when I apply for my license after an ignition interlock restriction?
You will need the Application for Driver License or Non-Driver ID Card, which is form MV-44.