Grease Peincil: How to do transparent masking

Imagine you have an eye, like

  1. Eye Iris
  2. Eye Shape (Mask)

You could use Eye Shape layer clip iris. Nice to have. But, I do like to keep Eye Shape layer transparent. Is there a way without workaround?

Found the problem. I used official Blender 2.82.

Today I tried newest Blender 2.83 build. The masking system has changed and exposes a ‘mask’ checkbox.
With this Blender build I was able to get a working transparent masking shape.