Don't know why you need a servo if you already have a steering servo in your car. You don't need it for the motor. Don't know what the "port" on the right is for or if it is even a port - there are two lines coming off already. And don't know why two lines. One is obviously the ESC input from the receiver; the other may be a separate brake input. Or an arming switch. If you can't find a manual on Amazon you might try HobbyPartz or Nitroplanes.
BTW, the photo of "waterproof" item is not the same item as the one on the box or being sold. Close but not the same. And there is no way that thing is waterproof. Water resistant maybe but not waterproof.