Thanks @ivan.m, I did it on my HeroBox. It works 
I guess you are my new hero now!
It was too small to solder on the power, so I just soldered to ground and taped the capacitor. That way it applies some pressure on the wire and it keeps in contact with the center of the button.
(on the photo you can see that I disconnected the ventilator, but it has nothing to do with the boot on power issue, it’s just too noisy, another bios issue ^^)