I have found that the simplest and most efficient option is to send an email to sbpcl@princesscruises.com and attach your account statement. Put your name, ship and cruise departure date in the subject line, then include the information you used to put on the form in the body of the email rather than add as another attachment. Princess seem to be processing via this method really quickly with OBC added to your account within a few days.
Hope this helps. 🙂