How do I get the output(Y) (on a subform) of a combo-box(X)(also on subform) lookup into a field(Z) on the main form?

I don’t want to lock the data in Z, though.
The user should be able to type over it if they wish. Z is search criteria which can be looked up and/or entered.

Example: Z should hold a highlighted 0000331146 if thats the value coming out of the combo box, or it should be null if the combo box isnt used. Please Help!!!