Hi @Lee Street
How are you indicating a "PO is added" to an issue? Let's say it is a custom field; you could try something like this:
project = myProject
AND Status = Backlog
AND "Story Points" IS NOT EMPTY
AND "PO" IS NOT EMPTY
ORDER BY Key ASC
For a Company-Managed project (formerly known as Classic) you could add this a quick filter to the board, allowing quick filtering on the Backlog view.