Button programin C++

[tomy]

so my team is using mecanum wheels this year. we would like to make it so when you hit the trigger on the left joystick it reduces the maximum speed of the motor by half. so im my code i have this

 bool GetTrigger (Joystick hand = leftstick);

if im right that checks if the trigger on the left joystick it pushed or not.
i was trying to create an if statement to change the speed of the motors but its not working.

any suggestions?