Populating A Field Automatically OR Allowing Manual Entering Of Data

I have a simple table with two fields in that table called:

1. BankName
2. BankNumber

Each bank has a bank number. For example Bank XYZ and all of its branches have the same Bank Number 123. There are 5 banks I have listed in a combo list under the BankName field. I can also type in a different bank in that same field if it is not listed in the combobox list.

Now, I want the BankNumber to automatically populate based on what I choose under BankName. If the BankName is manually entered (for banks that are not in the combobox), or if the BankName field is blank, I want the BankNumber field to be able to enter a number manually.

For example, if I go to the BankName field and under the combobox I select Bank XYZ, i want the BankNumber field to automatically populate as 123. If the BankName has a bank name that was manually entered, i want BankNumber field to allow me to manually enter a number.

Thanks for ur help. I couldn't figure this simple request out.