How do I create an invisible mask?

I am trying to make a mask so that sprites only show in a certain area. I know how to do this, and I can make the mask work with the sprites using Source In.

However, the problem is that the mask needs to have color to work. If the mask is invisible or has a transparent image then the sprites don't show. Is there any way to make the mask still function while not having any image on the mask?

No, the mask should have opaque pixels to work. But with the right blend modes it will be invisible to users.