Filter subform items based on parent form

Filter subform items based on parent form

I have a purchase order form with a subform for adding items to the purchase order.

I only want parts that belong to the vendor selected in the parent form to be available in the subform lookup field.

Each part in the subform is related to the vendor selected in the parent form.  

How can I apply a filter to the lookup field in the subform based on the vendor selected in the parent form?


I'm trying to use the Set filter in the field properties section but maybe this needs to be done with deluge.  Please help or direct me to something that might help.

Thanks