Reference number (PO) or CIF/NIF number on your invoice

Question
How to add a reference number or PO number on the monthly invoice?
Answer
It is possible to add a reference number (PO number) or a CIF/NIF number on the invoice.
This option is available via the control panel:
Account > Financial > Custom invoice information fields 
Or via this direct link:
Invoice Settings
After adding the information and clicking Save changes, these numbers will be present on all future invoices. In case the next invoice needs to contain a new / updated number, make sure the information is updated in the control panel before the new invoice is generated (1st of the month)
Please note that we are not able to re-generate past invoices to include these numbers.