To get the gnome shutdown dialog directly via a keyboard shortcut, add this command to the Keyboard shortcuts (under System -> Preferences):
/usr/lib/indicator-session/gtk-logout-helper -s
Give it a suitiable name, like Shutdown, and then map it to the Keyboard combination of your choice, by clicking under the ‘Shorcut’ column (which will be showing “Disabled”, until mapped). I just mapped it to Windows Key + End.
Now, enjoy shutting down your system!
For 12.4 lts , simply click the system settings(the spanner &bolt icon) ->click on keyboard icon->simply enter a suitable name and command of your choice
Thanks for sharing!