For a switch, you only need to understand how it works.
On the mobo there are two pins for the power switch (+ and -)
When you press the button, these two pins get shorted for that short time period, starting up the motherboard. (You can start it without a button by just touching the two pins with, say, a screwdriver--connecting the circuit for a split second.
So all you need are two wires (one on each pin) leading to a switch or button that must be held down to remain active. Tap the button once, the circuit is completed and broken immediately after, and your computer starts.