How can I add a PO number to an invoice

It sometimes happens that a customer asks you to add a PO number to your invoice.
The PO number should be added on Order (root)level in the property Reference.

Where to put in the header part of the Json:

{
 "OrderType": "Invoice",
 "OrderDirection": "Income",
 "OrderNumber": "QS-004",
 "OrderDate": "2025-05-05",
 "ExpiryDate": "2025-05-30",
 "Reference": "45123456789", //PO Reference of the buyer
}