1. Go to Settings > Invoices > Preferences and enable the slider “Use PO numbers on invoices”.
2. Create a new invoice or open up an existing one.
3. When your create or edit an invoice, you will find the PO number field there OR you can click the arrow on the top of the invoice page next to "Info" and select Enter PO number to add it.
4. To add the PO number to your invoices, go to Settings > Document layout > Invoices and add the shortcode $INVOICE_PO_NR$ to your Word template.
Note that you can also add the PO number as a column to your invoice overview by clicking the little cogwheel in the lower right corner. The PO-number is also available for exports to Excel.