You can easily add or change the invoice reference/PO-number on my.rackfish.com. Log in as usual and then go to Edit Details, and you will find the field at the bottom of the page. Remember to hit "Save Changes" when you're done.
Remember to change the invoice reference before the next invoice is generated. Retrospectively changing an already created invoice is only possible by contacting us at support@rackfish.com and is subject to a fee.