There's a bit of background work you'll need to do first to get this to work...
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install Nautilus-Admin
sudo apt install nautilus-admin -
set up sudoedit
The proper way to edit files as sudo
Now, open Nautilus, navigate to Other Locations > Computer > etc > polkit-1 > localauthority
At this point, a dialog box will pop up asking for your password... that's Nautilus Admin granting you permissions to access the directory.
Now navigate to 50-local.d . It should show Folder is Empty.
Now follow the directions here:
Create a file named /etc/polkit-1/localauthority/50-local.d/allow_all_users_to_shutdown.pkla and edit it using sudoedit to look like this:
[Allow all users to shutdown]
Identity=unix-user:*
Action=org.freedesktop.consolekit.system.stop-multiple-users
ResultInactive=no
ResultActive=yes
Then create another .pkla file in the same directory. Use any name you like ending with .pkla, for example, allow_all_users_to_restart.pkla, and fill it with these contents:
[Allow all users to restart]
Identity=unix-user:*
Action=org.freedesktop.consolekit.system.restart-multiple-users
ResultInactive=no
ResultActive=yes
Now go to Zorin menu > Settings > Power, and set Power Button Behavior to Power Off.
Now, all they have to do is press the power button, and the machine should power off.