PurchaseOrder Detail - set UnitOfMeasure default - correct event?

Our PO team wants Purchase Order lines to have the UnitOfMeasure to populate as EACH. I put in the most basic script I could think of;

retVal = oBusObj.SetValue("UnitOfMeasure$","EACH")

but I cannot seem to find the correct event to use.

I was able to get it to work on column pre-validate, but the only problem was that did it not only set the default I can't change it. Using the set-default event didn't work.