Can not adding camera component to script that attached on my prefab

Hello friends, I'm going to make ui buttons as move gameobjects. For this i use billboardscript for my prefab's canvas. (shown in pic.) When i open my prefab, i select canvas to add camera to attach script, the pop up window always empty. When its empty objects not look camera. I want spawn this prefab with this feature. what can i do ?

Code (CSharp):

1.  public Camera cam;
2.  
3.  
4.     void Update()
5.     {
6.  
7.         transform.LookAt(transform.position + cam.transform.rotation * Vector3.forward, cam.transform.rotation * Vector3.up);
8.  
9.  
10.     }

 

SOLUTION : WE CANT ADD CAMERA AS I SAID. I WILL SOLVE PROBLEM LIKE :

ADD CAMERA.MAIN WHEN OBJECT SPAWNED.