Turning on a PSU from a Pi.

I would suggest using a relay controlled by the Pi to close the psu circuit.

The amount of power that could end up running through the ground of your PSU would be far more than your Pi could handle, more than likely destroying the Pi. Consider using a relay, controlled by the GPIO of the Pi, to open and close the circuit. A relay is just an electrically controlled switch, but it's good for letting low power things like a Pi control higher power things like a PSU.

https://www.raspberrypi.org/forums/viewtopic.php?t=180424

This would be overengineering a very simple problem, you just need a button. GPIO's output digital signals which would be completely ignored by the PSU at best or cause damage (very, very unlikely, but I don't think impossible), so you need to physically bridge the green to a psu ground instead.