You need to apply 12 volts across the two small solenoid wires to cause it to close. One side ties to ground, and one side side ties to battery power. But this is usually done through the fuse, ignition switch, brake switch, and start button (and all associated wiring - including the ground return wires).
Study the two following diagrams (the two most common ways of wirng up the starter activation circuitry) and see if you can trace out the entire wiring path from the positive battery terminal to the starter solenoid actuating coil, and back to the negative battery terminal. Then use a meter to follow that path (ignition on, brakes applied, and start button pushed) and see where the voltage stops along the path.
Your plan of taking the two small wires on the solenoid and shorting them together gaurantees the solenoid cannot ever have 12 volts across those two wires, and so will not work.