Ubuntu instant-off option

Objective: (safe) power-off  the computer with pressing the power button

Refer to:
https://askubuntu.com/questions/430776/how-can-i-modify-the-power-button-behaviour
https://askubuntu.com/questions/1000393/how-to-configure-the-power-off-button-to-just-power-off-instantly-in-ubuntu-17-1?rq=1
https://ubuntuforums.org/showthread.php?t=2270015