Comment # 11 on bug 905159 from Bernhard Wiedemann

(In reply to Marcus Moeller from comment #10)
> @Bernhard, as ConsoleKit is deprecated anyway, I would like to suggest to
> stick with systemctl reboot and systemctl poweroff instead

this is interactive in a terminal - not sure that would work as an icewm
shutdowncommand (maybe would work with xdg-su)
systemctl reboot
==== AUTHENTICATING FOR org.freedesktop.login1.reboot ===
Authentication is required for rebooting the system.
Authenticating as: root
Password: 


will try this one next:
> ShutdownCommand="dbus-send --system --print-reply --dest=org.freedesktop.login1 /org/freedesktop/login1 org.freedesktop.login1.Manager.PowerOff boolean:true 2>&1 && return"