Populate drop down field from another form's subform

Populate drop down field from another form's subform

Hello,

I found how to do that, but not in case of a subform.

I have a Product form that has a subform for unit and prices. A product might have more than one unit. For example, the product "Brocoli" can be sold in unit at 3$ or in box of 10 at 25 $. Both prices and units are stored in a subform.

In an Order form, when I select "brocoli" with a lookup field, I want a unit & price drop down field to offer me the 2 possibilities from the subform of the Product form.

How?

Sylvain