Hi Evolution,
You can customize the template to include the PO number in the desktop version of the software, but this option is not available in the web version. The online invoice link includes the PO number by default. Quotes do not include the PO number on any platform because an order is only created once a quote has been accepted.
If you want this feature in the web version, you can suggest it on the Ideas Exchange board for other users to vote on for future updates.
Let us know if you need any other help.
Thanks,
Genreve