help with lookup and adding new record

[ma t]

Hello all, i am going in circles, again, I would appreciate any help I can get. I have a query with the fields Adult Last and Adult First from the "Adult Info" table. I have a form based on the query. I have been able to create lookups in the last and first fields to determine if the person already exists. If the person does exist, I would like a 3rd field, "adult ID" to return the person's existing ID (from the Adult info table) and if it's a new entry, return the next available number in the adult ID sequence. I greatly appreciate any help!