Access – form to sub form automatic entry

I’m trying to refine my Invoice Tracking program to have to vendor name automaticly entered on the parts sub form. Made a text box on the subform and clicked on the table line in the data box to the vendor table on the sub form. Then tried [Form]![Name of form]![Combo Box9]in the property box under data validation rule. Even the expression builder doesn’t come up with a formula that works.

Any ideas?