Figured it out, but my goodness was it difficult to track down…
Turns out you need to set “OrderAllocTableset.OrderAlloc.SelectedForAction” to “true” AND set the Row Mod to “U” for GetLotBinOnHand to recognize that the demand is selected. I am vaguely aware of what Row Mod does so I guess that makes sense. I had the first part figured out for a while, and after comparing lots of traces I was able to deduce the Row Mod part. However, then it was not obvious how to achieve that in a Widget function.
Unfortunately, the “Set Field” widget does not automatically (or provide the option to) set the Row Mod to “U” (RowMod is hidden from the column selections for some reason?). I would think this is a fairly common requirement?
This post gives a work-around for Row Mod using Widgets (using “Update Table by Query”):