If you mean you want to jump start motherboard you can find the 5 pin power switch header (2x3 pin with one blank, right next to DIMM slots) and short 2 pins momentarily using flat blade screwdriver.  990/790 have same 5 pin. [IMAGE] [IMAGE] (5 pin pwr sw at upper right corner)
how would i go about powering on this device till i get the power switch 
optiplex 990