Hello,
In older gnome version it was possible to do so with dconf app. I could just press power button and system turned itself off right away. It was so convenient - no annoying dialogue box that it will turn itself off in 60 seconds - just two cliks with mouse or one with power button.
Is it possible to do it now?
Thank you
Hello? m? 
So why, oh why? And how? m?
I use an extension and also helps by making an hibernation button. One click to restart or hibernate.
I use it as well, but there is no more option to enable instant shutdown like it was before or maybe because now I installed Silverblue 39 and it was hidden.
What operating system you are using? is it GNOME Shell 45.2?
Using Manjaro latest stable
Set a keybinding in settings to run this command:
shutdown -P now
be careful though you might lose your unsaved works after pressing keys
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it works if sudo is added at a beginning, but it would be better if it could should down with one power button press. 
What’s the point?
You can hold the power button until it’s off but it might leave some programs unfinished or changes unsaved.
Or you can choose shutdown when pressing power button even though you have to confirm before shutting down… Or wait 60 seconds for it to shutdown itself
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It is not convenient.
Now I am powering off by pressing power button > left button > enter. Three buttons instead just one! Maybe it is good for finger training. I can do it very fast