I am creating a simple sales, product and inventory app.     Issue is each product has multiple sub items to track and update

Forms:

Products

Boxes Inventory

Sales

My "Sales" form is calling the  "Products" form to get a SKU and what box it is shipped in.  Based on the box it is shipped in I need to go to my "Boxes Inventory" form find the box and subtract quantity sold.

 

What I don't know how to script is the connection to the "Boxes Inventory" form and the update to the box quantity on hand field.  There is no lookup field connecting the two forms. I only have a value in a field passed over from the "Products" lookup field.