We use the "Order Items" screen to create PO's based on MRP Planned Orders and we need to preassign some values for those PO's. • How can I identify within a workflow that a PO is being created based on a Planned Purchase Order?

Once created, does the Purchase Orders have a relationship with the Planned Order? Like on the field Created From or similar?

if it has some sort of relationship then you can use a saved search with that criteria in your saved search

Yes, under the PO Related Records I can see the Planned Purchase Order, but the "Created From" field is blank. I was planning to validate that filed on a on create workflow for the PO, but it is blank.

Maybe the relationship is at the line level. I would test it using Saved Search, worst case using sql and the previous/next transaction line table.