To enable paying through a purchase order, you need to add an invoice option to the checkout page:
Open the Checkout Layout that you are using on your storefront. (The default is Checkout Layout with GiftCard.)
Switch to grid view.
Add the Payment Gateway widget to the layout.
Add the Invoice Payment element to the Payment Gateway widget.
Publish your changes.
If the invoice gateway is enabled, the checkout page displays a checkbox for optionally requesting an invoice. When the shopper selects this checkbox, a text field appears for specifying a purchase order number:
