Hello,
I don't know what you mean by "custom PO" and you certainly don't have to create a new workflow in SWDD for every PO!
I advise you togo to your Test system and turn on the event trace (SWEL) using tx SWELS.
Create a multi-line PO in ME21N and save it. Check in SWEL to see which events have been created.
These are the events you can use to trigger your workflow. You only have to create it once in SWDD.
After that every new PO will create a (or multiple) workflow instances.
regards
Rick