While im sure there is a way to do it, i would like to offer a very very easy way to just set a button directly through your motherboard. It will require slight wiring, but nothing crazy.
On your motherboard, locate where the power button runs to. Should be 2 pins somewhere on the board. Those two pins just need the connection between them completed to power on and off. You an test this by simply touching a screwdriver head to them both together and it will power up. Assuming youre currently powering up with some sort of button, you can do a few things.. You can use the wire and button already used on your motherboard..My button was a tiny button in a square plastic piece, so i simply glued it onto a pushbutton, where the plunger would hit it when pushed, and left it connected to the 2 pins..That was simple. The more official way would be to just strip the end of the wires, and run them to a microswitch, one wire to the ground one to the nc switch. If you dont have a button currently running to the motherboard power pins, you can use a lot of different wiring from a old pc.. the led lights, or anything. You just need 2 wires running to a connector to sit on the 2 pins, and the other end running to a microswitch or any other rigging of a button. I hope this helps. i typed it from my phone or else id include pictures. Im sure someone can add a more proper explanation with proper terms for the switches and such.