....

//define the custom motion control function
// custom PID controller instance for the custom control method
// P controller with gain of 1.0f, no integral or derivative gain
PIDController custom_PID = PIDController(1.0f, 0, 0);
// custom motion control method
float positionPControl(FOCMotor* motor, float target){
  // simple proportional position control
  float error = target - motor->shaft_angle;
  // set the PID output limit to the motor current limit
  custom_PID.limit = motor->current_limit; 
  return custom_PID(error); // return current command based on the error
}

... 

void setup(){
  ...

  // set control loop type to be used
  motor.controller = MotionControlType::custom;
  motor.torque_controller = TorqueControlType::foc_current; 
  motor.linkCustomMotionControl(positionPControl); // set the custom control method

 ...