1. Go to Settings > Invoices > Preferences and enable the slider “Use PO numbers on invoices”.
2. Go to Invoices and click on the plus sign to create and book (!) an invoice OR open an existing booked invoice.
3. Click on the arrow at the top of the invoice next to "Info" and select Enter PO number to add it.
4. Go to Settings > Document layout > Invoices and add the shortcode $INVOICE_PO_NR$ to your Word template.