Kmail and toplevel-tag/Wayland

Excuse me if this isn't the correct place to ask this question.  If not, could 
you direct me as needed?

I'm using the latest Plasma/KMail from Neon, and I'm wondering about the 
Wayland toplevel-tag extension implemented over the past year.  Specifically, 
how does one use these tags in KMail?  I tried selecting "Window Tag" in the 
window rules, but it doesn't show any options.  I assumed it would show 
something like 'mainwindow', 'composer', etc, like it did under X11.

Thanks for any info you can provide.
Steve A.