Turning on a 300w computer power supply.

[mdanh2002]

I would use a 12V automotive bulb such as SAE1156 single filament stop light on the 5V line. It won't glow real bright, and won't get unbearably hot.

I usually connect 1 or 2 CPU fans to the supply, and that's enough to keep the ATX supply on

 

[eggplant_casserole]

Short pins 13 and 14. Job done. (Yes, green to ground in other words.)

Have you tested the supply to see that it's good? It really needs testing under load - we found that even with an ATX tester, because it wasn't under full load a PSU could be reported as good, but wouldn't work when used in a system.