Probuilder - How to snap face along one axis?

I'm using probuilder.

I have a set of stairs, and then a cube to act as the platform at the top of the set of stairs.

I cannot for the life of me work out how to snap the face along the one axis to match the height of the stairs, and yet I'm sure it must be possible.

Of course, if I use 'V' for vertext snapping, the whole face moves so that the middle of it snaps to the corner of the top of the stairs. What I want is for it just match the height along the y-axis so the block is exactly the same height as the stairs.

How do I lock things so there's only manipulation along one axis?

Does that make sense?

 

I've had a number of views but no answer yet. Is it because what I'm looking for isn't possible, or is it because I've missed something really obvious?

Trust me, I've looked in the ProBuilder docs and can't find the answer. I know ProBuilder *isn't* a full-3D program, but I'm certain this functionality must be there.

 

That's really helpful. I needed to see you do it with the cursor to make sense. Thanks very much!