When creating a weekly invoice you can enter any Notes you’d like to include on the invoice. (For example, some Suppliers include: We appreciate your business, or when you need to include a PO #, enter the PO # in the field as seen below. Only copy an individual on the invoice if you are instructed to do so by that individual. (There is already an Accounts Payable recipient email configured in the system and most Buyers prefer that A/P is the only one who is emailed.)
The attached video (a little over 2 minutes) provides instructions on where/how to include the PO number on an invoice.