Resetting counter on Autonumber text box (Primary Key)

I am creating a file in which I would like the record key to be generated by AutoNumber in the Data Type property. I have successfully created the file with this field. How do I reset this counter to 1000 on an empty file? I use this file for invoices and this will be the invoice number.
It wants to start at 1 and I want to plug in some existing invoices.