Not sure if this is possible, but try purchasing using whatever AC.com provides as a routing and apply your discount code. Then call AC and request a routing change. I understand that you can cancel within 24 hours, so you can always try this and if not successful simply cancel your online booking.

Keep in mind that most of the AC discount codes do not permit multi-city bookings (which I interpret to meaning stopovers) so you technically shouldn't be able to change to a routing that changes your ticket to include a stopover.