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.

Adding empty order lines can help create a clearer invoice. These can be either for spacing or to add a description. They can be added as follows. Keep in mind that some e-invoice networks might create a zero line from them with 0 excl and 0 VAT%.

{
  "Reference": "PO-Number" --this should be supplied by your customer
}