Customizing Customer Maintenance Form

I have a user defined table UD20 with
StateDescription(Character02), StateCode(Character01). I created a
unicombobox that will bring up the StateDescription. I would like the
user to select the StateDescription and move the StateCode into
the "txtState" Field on the form. I have the combo box next to the
state field.

Can someone show me the code to make this happen??

When I use the event form wizard the event table, and fields do not

Thank you