These forms usually have a current or desired salary field and often has fairly strong validations on it (e.g., must be filled in, must be a number between x and y, no non-numeric characters allowed).
So you either have to fill it in or not submit the application form at all (the rest of which is usually perfectly reasonable), which is not going to impress anyone.
I suppose you could also hack their javascript and take out their validations and hope they don't have server side validations as well. That should impress them :).
edit for sense/grammar