iProcurment and Blanket Purchase Agreements Link?!

Hi,

This may be simple to some but I am stuck.

So I want to assign a Blanket Purchase Agreement (BPA) to Procurement catalogue, so when purchases are made in the store for certain items it will go against the BPA.

How do I link a blanket purchase agreement to a procurement store?

I know you can put contract numbers in the store setup page, but I cant find the BPA number when I type this. What am I doing wrong? Is there another way to link the catalogue item and the BPA that I'm missing?

Also, I've got a BPA linked to a store item already, but I cant find how to link these back!