Adding key switch to a PC.
Most (if not all) motherboards are built in such a way that they require a momentary connection to start. The board is under power, and a controller is checking if the power button is pushed (and released again). When it senses this, it will power on the system. If it senses a new push, it will tell this to the processor in some manner, so the operating system can decide how to deal with it.

This means that you can't use a classic "on/off" switch can be used on it's own.

However, as the video demonstrates, nothing stops you from using it in series with the original switch. In your case, you could use an inverter in series with the original switch, with the inverter connected to the switch. 

You would need to find the required voltage rails on your motherboard to do this, and watch out since motherboards can be fragile, and mistakes can lead to damage to attached components such as the CPU, PCIe cards, ...