You need to know it, there’s no list directly on the field. Alternatively use the programme field and use the list and this displays the UCAS code alongside the MAS (Marketing and Admissions System) code.
Provided the applicant status isn't rejected you can. To do this: in the View/Process Applications screen, retrieve the applicant, go to the Decision button, tick the withdrawn checkbox. Change the Processing status to FWD, make sure the two offer boxes and the offer note box are blank. In the first offer box type the code from the offer library that describes the reason for withdrawal:
W1 = Withdrawn at own request
W2 = Withdrawn - Did not attend interview
W3 = Withdrawn - Did not reply to letter
W4 = Withdrawn - Course not available