Mask Units in a stack but not the top unit

Module Design
1

I am design a module for Eagles of France and I would like to use a button that mask all units but the top unit in a stack.

How do I define that the unit in the stack is in the top and no affect the order?
Thanks in advance.

2

See the StackPos property on the properties section of the Basic Piece definition.

3

Thansk @jrwatts but I don’t know how use StackPos u pus match property StackPos = 1 but not work.
Would I have to define this property previously??