OK, but how you're gonna do it will depend on if your chassis is GM or Ford.
GM - should be battery power on the heavy cable attached to top of solenoid. Jump it to that small stud on the engine side of the solenoid (as MEX explained) and it should crank.
FORD - that heavy cable is NOT powered till another solenoid that looks like this
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On this solenoid, Battery Positive Cable goes to one of the large terminals and the cable down to the starter goes on the other. Jump the small terminal you can see in the pic, to the terminal with the cable from the Battery, and it should send power down to the Starter Motor.
If FORD has a solenoid ON the starter, it's connected in a way that it will engage as soon as it sees power on the big cable. No separate energizing down there. Only to the one I pictured.