What type of scripting event does creating a PO automatically from a dropship SO trigger?

I am trying to get the form changed that is used when a dropship sales order is approved. I understand that the purchase order is automatically created if preferred vendor is on the item page, and I like the way that entire process works. However, I am attempting to write a script to change that purchase order form if the purchase order is from a specific vendor, but I cannot seem to pin down how to trigger the script. It seems as though the automatic creation of the PO does not trigger any of the User Event or Client script