Thanks.
What I did was:
I used the ribbon cable and unplugged it on button side (needed to unscrew that one screw in the metal and bend it back a bit, then force the cable out, it is sort of self locking and I am not sure if i can manage to get it back in there with tweezers, maybe yes maybe no, but i don’t need that button anyways).
As one can see on the connector side on the mainboard, three of the six lines are not connected to ground, there is a square on the motherboard. I used those lines on the other end of the cable, soldered a fine wire just onto the ribbon. I think i only hit two of the three contacts but it works fine. Now if I ground that cable it starts. Thanks Dan!
Didn’t want to solder on the mainboard. Now at least I have the chance to undo everything without it being noticed ;)