This has been requested for years and years Russell.
You can add a single custom box in the custom field section and select which form you wish them to appear on.
Intuit has issues with adding really basic elements, although they call them features, to their templates. I typically have multiple PO's per invoice and the best approach (although it's very ugly on the form) is to use a blank line with PO: ###### above each invoiced line. I still hate it but it's the only way of doing it really if you have multiple PO's. Easy fix by Intuit would be to add a column to the left of the form, like we used to have in Quickbooks Desktop for the previous 25 years.

