Auto Updating Sub form based on a Lookup input that is not in the sub form

Auto Updating Sub form based on a Lookup input that is not in the sub form

Hi Guys, 

I need some assistance. 
I have two forms: 

01. A purchase request form that has a subform with details of items being requested for. The form also creates a unique number for each submission (the PR_Code). 
02. A Purchase Order form that has a lookup field for the PR_Code. This form also has exactly the same subform as the PR form, but with extra fields for price and total price. 

I want it to function in such a way that when I look up the PR_Code, It automatically fills the subform of the PO form except for the price fields, if possible

thanks a lot