If you want a customer's PO number to appear on a stamp, for instance on FEDEX, You need to add it manually to the "Custom Field 3". You'd be shocked to find out that you cannot set up an automation for this.
What should happen is, you should be able to have an automation rule where you could have "custom Field 3" auto populate to match whatever the PO field says so you don't have to do it manually every time.
I doubt this would take an engineer more than 10 minutes to set up. Nor can I imagine I'm the only one who would like PO#'s to show up on a stamp.