@Victor2713 Probably the easiest thing to do is to make the offer code available at the event, and to tell folks that if they message you and include the code, they can get a discount. Then you can do a "special offer" back to them with the discount included. It seems a bit cumbersome, but I don't think AirBnB has the capability you are looking for, and this seems like a way that you could track how effective your advertising is. You can make the code specific to the event, so you can distinguish between multiple events you've sponsored.
Cheers, Kia