The best solution is to wire up a couple of switches to a 15-pin connector and plug it into the competition port. See the
Competition Port Pinout Guide on the
IFI Robotics web site.
You can also write your program to call the autonomous routine when a button is pressed on the Operator Console, but that's not very convenient. You would need to change the
while (autonomous_mode) statement to also check that button, and you have to be very careful about assuming anything about the state of the OI input values.