The invoices produced by Hop will always display a reservation number or House Account number, as applicable. If you wish to add another reference number to a reservation then the PO number or Payment Terms fields would both be options here.
The PO number field is available by clicking on Edit Reservation in the drop-down menu next to the RES number, or by clicking on Edit House Account in the drop-down menu of a House Account. If you fill this in and then generate an invoice it will be pulled through and shown on the right hand side of the invoice, as follows:
The Payment Terms field has the benefit of being a larger size to allow more text and can be updated on a reservation either from the transaction screen or when editing the reservation:
On house accounts, you can only access the Payment Terms field by going to the transactions screen and clicking on the Show Payment Terms button:
This will then bring up the same box as shown above in a pop-up window.