How to Invoke a function in subform.

How to Invoke a function in subform.

I have created two forms shipping and product Info and embed the product info form into shipping form. There are no relationship between shipping and product info form. I have created a custom function which get the product id and fill the unit price. When I invoke the function on user input, it doesn't execute the function and gives and error product_id doesn't exist. When I run Product Info form individually, It is working fine. But when I embed it to another form it doesn't work.
Please tell me how can I invoke function on sub form.