unet : modify PlayerPrefab in the code

Hello, I'm try to make a multiplayer game but for this i need to change the playerPerfab of the NetworkManager. I try to use the OnServerAddPlayer override function but this doesn't work, my code :

 public GameObject[] Players;
 public GameObject Select;
 public GameObject SpawnPoint;
 public int Counter;

 // Use this for initialization
 void Start () {
     Counter = PlayerPrefs.GetInt("Counter");
     Select = Players [Counter];
 }

 public virtual void OnServerAddPlayer(NetworkConnection conn, short playerControllerId)
 {
     var player = (GameObject)GameObject.Instantiate(Select, SpawnPoint.transform.position , Quaternion.identity);
     NetworkServer.AddPlayerForConnection(conn, player, playerControllerId);
 }
 
 // Update is called once per frame
 void Update () {

 }