How do I add a Purchase Order to invoices?

You can add a P.O. number during the checkout.

By default, the field is only displayed on Checkout when you are updating your subscription (e.g increasing your plan limits such as storage capacity and seat count).

Where is the entered P.O. number displayed?

You can find the P.O. number for each invoice in the Invoices dialog. You can open it by clicking on the Invoices button in the Billing tab of your Workspace Settings.

The purchase order number will be displayed when you hover over each invoice/receipt.

You can also find it if you download the receipt/invoice as a PDF: