Applying Business sign-up credit

[Merv1]

I just booked a room for an upcoming business trip, and only /after/ I requested it did I see the promotion to sign up for a Airbnb Business and get $50 credit.  So I signed up.  Then my soon-to-be-host accepted my reservation and I'm all set.  

HOW do I apply that $50 credit to the trip I just booked before/while I signed up for Airbnb Business?  I want to retroactively mark this trip as a business trip and get the credit.