I looked around and didn't found anything on this, for what I've seen, people always have been satisfied with what logind.conf is offering, here is the interesting part of
HandlePowerKey=, HandleSuspendKey=, HandleHibernateKey=, HandleLidSwitch=, HandleLidSwitchDocked= Controls how logind shall handle the system power and sleep keys and the lid switch to trigger actions such as system power-off or suspend. Can be one of "ignore", "poweroff", "reboot", "halt", "kexec", "suspend", "hibernate", "hybrid-sleep", and "lock". If "ignore", logind will never handle these keys. If "lock", all running sessions will be screen-locked; otherwise, the specified action will be taken in the respective event. Only input devices with the "power-switch" udev tag will be watched for key/lid switch events. HandlePowerKey= defaults to "poweroff". HandleSuspendKey= and HandleLidSwitch= default to "suspend". HandleLidSwitchDocked= defaults to "ignore". HandleHibernateKey= defaults to "hibernate". If the system is inserted in a docking station, or if more than one display is connected, the action specified by HandleLidSwitchDocked= occurs; otherwise the HandleLidSwitch= action occurs. A different application may disable logind's handling of system power and sleep keys and the lid switch by taking a low-level inhibitor lock ("handle-power-key", "handle-suspend-key", "handle-hibernate-key", "handle-lid-switch"). This is most commonly used by graphical desktop environments to take over suspend and hibernation handling, and to use their own configuration mechanisms. If a low-level inhibitor lock is taken, logind will not take any action when that key or switch is triggered and the Handle*= settings are irrelevant.
I then repeat the interesting part here:
Controls how logind shall handle the system power and sleep keys and the lid switch to trigger actions such as system power-off or suspend. Can be one of "ignore", "poweroff", "reboot", "halt", "kexec", "suspend", "hibernate", "hybrid-sleep", and "lock".
Or I am in the wrong way and this is just for keyboard keys, and not power button?
In any case, previously, it was easy with acpi, one just had to replace the power_button script in
/usr/lib/acpid/, isn't there something equivalent for systemd ?
NB (IMPORTANT): How can I run a script on keyboard power key press with systemd? is NOT a duplicate since has been wrongly marked as duplicate of How to change Power button shutdown action to run a script under systemd that DOES NOT answer my question, since this is to manage power key from keyboard, not power button:
And as suggested by @TooTea it may be true that button integrated into the case is seen as a keyboard button press, anyway, after having checked I have no such /dev/input/by-path/platform-i8042-serio-0-event-kbd file to monitor key pressed, then it definitively does not answer my question.