Hello HZN,
Unforunately, no - there's only provision for 12 characters in the reference. However, Adagio OrderEntry support 3 optional fields in the order header, up to 35 characters in length each. I'd suggest putting the last 12 digits of the PO # in the reference field, and using the optional field for the full PO # from your customer. You can print the longer number on their invoice.
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Andrew Bates