1. Go to Settings > Invoices > Preferences and enable the slider “Use PO numbers on your Invoices”.

2. Create a new invoice or open up an existing one.

3. When you create or edit* an invoice, you will find the PO number field there OR you can click the three dots on the top of the invoice page next to "Info" and select Enter PO number to add it.
*Note: When you've disabled the editing of booked invoices, it isn't possible to add a PO number anymore. Read more here.

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 or CSV. On top of that, invoices made from a deal or project will automatically take over the PO number filled in on that deal or project.