Collision object tag

Can someone help me with adding tag of object which can be this script triggered var rb: Rigidbody;

 function Start() {
     rb = GetComponent.<Rigidbody>();
 }
 
 
 function OnCollisionEnter(collision : Collision){
     for (var contact : ContactPoint in collision.contacts) {
             Debug.DrawRay(contact.point, contact.normal, Color.white);
             rb.velocity = new Vector3(-rigidbody.velocity.x, -rigidbody.velocity.y, -rigidbody.velocity.z);
     }
 }

i got script here and i want to add tag but i dont know where i could add it Sorry for bad english