The simplest way to trap a record commit is with an OnRecordCommit Script Trigger. The script should use an Exit Script step with a False result. This cancels the record commit.
(Be sure you leave a way out of the trigger, like a button that says "Save". Otherwise, you'll trap the user in an inescapable situation.)
.... or set the portal off screen in the right hand 'developers' area' (assuming you're using a more recent version of FM) - put an 'add' icon at the end of your [global] loader fields - attach a script to set the out-of-site portal row to the contents of your loader fields, then clear the loader fields. No leaving record required. Would just add an on exit script trigger to clear the globals if the user backs out without creating the record.