Turn on / test power supply without logic board connected?

Hi, is it possible to turn on the power supply by short-circuiting two pins or something similarly simple? I want to turn my old iMac into an external display by reusing the main case and Display (obviously) together with PSU, LED Inverter and speakers. I bought an LVDS-Controller and a mini 2.1 Channel amplifier for that. Now i dont want to buy a third party PS to power the thing, but reuse the original one and be able to use the original power button to turn everything on and off. Any suggestions?