Anytime an artist has an active promo code, there will be a spot to put it in, when you're filling out your info during checkout.  If there's no field for it, next to the Tip option, then the code you have has apparently expired.

If you'd like to use a code still, you should encourage the artist to ask their Booking Rep if they can get a fresh code, with a new expiration date.  They can sometimes extend offers or issue new ones.  It just depends on event.