You can advertise a discount and encourage a booking by:
1 - Updating your short description/title to note something like *Late booking special Extra 15% offered by host at booking"
2. If the guest books, you then immediately send them a note (if you haven't already answered this question), that you are sending a reservation change request that will discount their nightly rate, and that they need to accept.
3. Go into the "modify or cancel reservation function", reduce the total amount by the % or $ off discount you advertised. Send the request.
The guest accepts and the total is adjusted.