How to Fire a projectile using the right joystick(mobile assets) ?

hello, I'm in doubt .. I created a dual joystick that comes in the mobile assets and would like to how to put it in my code in the input for example "Input.GetKey (the name of my joystick)? I'm doing for my right thumb throwing a projectile then such

var fireTouchPad: Joystick;

function Update () {

  if (Input.GetButtonDown ("Fire1") && (time.time> lastShot fireFreq +)) {
     Fire (); "

I wonder if I have to put my right joystick instead of "Input.GetButtonDown"?

thanks for the help