Hi TRNC,
Yes most likely if the Parent table is a PO table then the Related PO table would most likely be the matching field between the two tables. If so you can use a Formula URL button set to use that Related PO field to display the Parent Record when the button is pushed. It would use a formula url field with a formula like:
URLRoot() & "db/" & "DBID Here" & "?a=dr&key="&[Related PO]
What you would need to input here is the DBID of the Parent table in the formula where it says DBID Here. The DBID can be found by going to the table you are wanting to display and in the URL in the address bar it is the chain of numbers and letters that appear after /db/ and before ?a=. That essentially lets the button know which table you are trying to open up and then asking it to display the record from that PO table that has the same Related PO as its Record ID#.