We represent multiple vendors as commissioned sales reps.  We track our sales by entering them on a Purchase Orders tracking form.  One of the fields on the main form is Vendor, a lookup from the Vendors table. 

The form has a subform to select the Product sold, and enter the cost and order quantity.   The subform field Product is a lookup from the Products table.  I would like to limit the list of Products in the dropdown to only those sold by the Vendor that was selected on the main form.  I can't figure out how to do this.  I tried creating an On User Input variable on the main form after selecting Vendor.  But when I try to set that as a filter on the subform I get the message "variable Temp.Vendor not defined." 

Any help would be appreciated.  Sorry if this has been solved before; I see where a number of people have asked this question in the past, but I don't see the answer to this specific question.