first you need to link the two tables throw a relationship, so the table "PO" should have a field called "id-client" (for example) which is linked with the the "id_client" of the table "Client",
and you could set a script and put it on button in the PO layout:
set variable ($idClient; PO::id-client)
go to layout (client)
set capture (on)
perform find (client::id-client; $id-client)
Using that same relationship suggested by Kays Khelifa, you can also place a portal to POs on your client layout to see a scrolling table of all PO's. A button inside the portal row can then be scripted to take you to that specific PO.