How to set the "Connected Body" of a HingeJoint 2D using Script?

At first, I add the hingejoint 2d component to the gameobject. Then, I have a prefab, I instantiate it and want to assign its rigidbody 2d component to the connected body of the hingejoint 2d. Iam unable to do this. I read all the documentation as well as the blogs. Please help. Here's the code.

 //Boxnew is the prefab, I want its rigidbody component
 GameObject box2 = (GameObject)Instantiate (boxnew);
     
 //Now I attach hingejoint to my gameobject, works fine
 hingenew = gameObject.AddComponent<HingeJoint2D>();
 
 hingenew.GetComponent<HingeJoint2D>().connectedBody = box2.rigidbody2D;    
 //Error Above
 //Error: InvalidCastException: Cannot cast from source type to destination type.

Iam unable to set the connected body at runtime. Literally gave up.