Where do I go to enter a purchase order into a customers file in Invoices, so it automatically appears on each invoice prepared for them, without me having to enter it manually each time?

I assume this is a "standing" purchase order.

You can create an optional field called "Standing PO" and fill that in as needed for individual clients.

Then you need to add it to your invoice specification. You may need your dealer/consultant to help you with that part.