Good morning Justin;Can you talk a little more about the version of AX you are using, and your current process? Are you using Free text invoices, originating from sales orders, or something else?Were on 2012R3, and I don't recall if we've customized around this: On both the FTI and Sales Order Invoice, the "Alt PO" is displayed on the invoice document.
Alternatively, you could utilize out-of-box attachments/notes. Look in AR->Setup->Forms->Form setup
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