...
void Start() {
  _camera = GetComponent<Camera>();
  Cursor.lockState = CursorLockMode.Locked; 
  Cursor.visible = false;                     
}

void OnGUI() {
  int size = 12;
  float posX = _camera.pixelWidth/2 - size/4;
  float posY = _camera.pixelHeight/2 - size/2;
  GUI.Label(new Rect(posX, posY, size, size), "*"); 
}
...
