Auto increment textbox

Hello, 

I have a textbox on a form called ReservationID, I would like to be able to assign a value to this field automatically. I have created a query which lists all of the existing ID, called Qry_ReservationID. I have this code under form_open so far:

I want the ID to be set to AB0000, and it goes up by 1.

The ReservationID field in Reservations is set to text.