How to copy/put values to subform?

How to copy/put values to subform?

Hello, 

I am trying to copy/put a value from a picklist to a picklist in a subform. How can I solve this with a function?

So in the Invoice module I want to choose a value from Picklist A (a custom module "Products") and I need to be copied/put automatically to the Picklist B (the same custom module "Products") in the products subform of the Invoice modul. 

Thanks for your help. 

M.