I found a way to forcefully turn on a motherboard with an unresponsive soft power button and power header

[Kaicer]

Note: Before doing this, I would highly recommend finding an old power supply that you wouldn't mind modding and cutting cords. 

 

Materials needed:

• Old power supply
• soldering iron
• Flux
• Solder
• Any kind of switch (not including light dimmers or potentiometers)
• 15-18 Gauge multistranded copper wire
• Wire strippers
• Wire cutters
• Heat shrink or electrical tape
• A trusty lighter

Process:

!Don't forget to unplug the power supply!

• Step 1. Locate the green wire and one of the nearby black wires on the 24 pin connector.
• Step 2. Cut the green wire and the black wire about two inches away from the 24 pin connector.
• Step 3. Strip the ends of the green and black wire that are coming from the power supply.
• (Optional) Step 4. Slide one piece of heat shrink onto both of the wires.
• Step 5. Solder the switch to the green and black wires.
• (Goes with step 4) Step 6. Slide the heat shrink to the switch and put the lighter underneath.
• Step 7. Wrap each end of the wire with electrical tape.
• Step 8. Plug everything like normal and flip the switch. 

[Blai5e]

This is the old jump start trick with a convenient (but permanent) variance - a switch. If your wires are not colour coded, on the 24 pin connector (looking at the pins) with the locking clip facing up it's the 4th and 5th pins on the top row.

[LukeNukem9]

Kinda like the AT power supplies.

[Kaicer]

7 minutes ago, LukeTheCoder05 said:

Kinda like the AT power supplies.

I have also seen this on an old Compaq Power supply. It had a push button hooked directly into the power supply by a long wire and then the button was put in the front of the case.