you need to use the combo notinlist event typically to update your payees table with the new value, or more likely open the payees form so the user can enter the payee details because you presumably have more than one field to complete - id, name, perhaps full name, type of payment, payee account details if paying electronically etc.
If your payee list is just a list of names and not intended for control/spelling purposes (like a list of US states for example), you can just use a SELECT DISTINCT query for the combo rowsource based on the table and field the combo refers to and do away with the payee table
e.g.
Code:
SELECT DISTINCT Payee
FROM tblTransactions
ORDER BY Payee