Very basic customisation query

My current workflow looks like:

Opportunity -> Quote -> Invoice

I would like to be able to add a purchase order number field to the Opportunity record, which I have done using Studio. But how do I pull this value through to the Invoice stage? I need to have that PO number printed on the final PDF.

At the moment my invoice PDF template is using the field $aos_invoices_description which is pulling values from the “Description” field on the Opportunity record. How are these fields related? And how can I replicate that relationship for my new PO number field?