Attach 3d object to screen

I want to attach an object to my screen. (like object stays always in the center of the screen) I have one camera. I am currently not moving the, but I can't position my object in the right place. Here is what i do:

 void Start (){
 //Find the camera
 go=GameObject.Find("ARCamera");
 
 // transform screen position to world position
 Vector3 vector = new Vector3 (0, 0, 20);
 Vector3 p = go.camera.ViewportToWorldPoint(vector);
 
 //transform the object
 transform.Translate(p,Space.World);
 }