Have you traced the methods yet? First you want to turn on tracing, then perform the actions in Epicor to add new PO records. The trace will record which methods fire. Turn off tracing once you are done adding the records. Then you should use Jose’s Trace Differ Tool (Trace Helper Utility for Epicor ERP 10) to more easily see the methods you need to include in your action.
Reproduce those methods in a custom action in your UBAQ. Do not use the base events, make a new custom action. You also have to define the action in the BAQ menu Actions > Define Custom Action.
Also check out this older post with tons of helpful code blocks and notes from people smarter than me! Create new line and release from UBAQ custom code
Good Luck!
Nate