Just like this:
runas /user:Administrator@mydomain.com "shutdown -r -m \\MACHINE_IM_RESTARTING -t 3 -d P:00:00"
runas = run as a different user, in this case admin.
shutdown options:
-m = Machine name with \\ in front of the name
-r = Reboot (not shut down)
-t 3 = Show clients a warning for 3 seconds
-d P:00:00 = Reason for shutdown 00:00 is more or less “because i felt liek it”
Example #2:
runas /user:falcon@norad.mil “shutdown -r -m \\WHOPPER -t 3 -d P:00:00″