How to set up supplier site when generating an RFQ using the isupplier portal (R.12.2.9) ?

When creating the RFQ, it expects to show the supplier site from the Approved Supplier List created, which has a supplier and site combination. But for the user, when creating an RFQ from the isupplier portal, the supplier site is not picked as the default. The user has to manually select the supplier site when adding the supplier to the RFQ. How can I set up to default the supplier site in the iSupplier portal?