Unfortunately this is another feature that was available in the Desktop version but that they have removed from online.  Having it come from a quickbooks.com is a nice free advertisement for Quickbooks and obviously that is more important than your invoicing looking professional for your company.

Naturally they don't tell you about this before you move over just like they don't tell you you will have to upgrade to a higher version of QBO if you want to add something as obvious to an Invoice as the associated PO number.  After all, who on earth would expect to be able to have the PO number appear on an invoice??  That is just crazy talk.