The first thing to do is go to Settings > Invoices > Preferences and set the “Use PO numbers on invoices” preference on 'Yes'. Whenever you create and book (!) an invoice, you can click on the arrow at the top of the invoice next to info and choose “Enter PO number” to add it. The only thing you still need to do in order to put the PO number on your invoice is to add the shortcode: $INVOICE_PO_NR$ on your word template.
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