It looks impossible, but I'd like to create a dependent dropdown by setting a criteria in a Lookup such as
a) field_fky == input.field_fky
rather than the only supported method that requires an on input action on the field
a) would be faster to create and the resulting field would appear in views, unlike b)
What happens when I try a) is an error message that "this." is not in the form. When I look at the API for dropdowns, it's seems that the syntax <formname>[criteria].<fieldname> is inherently limited to references to fields on <formname>.