Create autonumber in textbox to store in table

Hello,

I currently have a ReservationID field in a Reservations table, I need the ID to be auto assigned, however I also have to put in a range of my own ID values in this field. e.g. I need it to start from 1, but I have values such as 200 and 400 which need to be in this field as well. How do I go about resolving this? Can I just set the ReservationID textbox in the form to create the incremented numbers?