offset on Configurable Joint target rotation

Hello, I want to have an offset (-90 on z) on the rotation. How can I do that? This is my script:

 public class CopyMotion : MonoBehaviour
 {
     public Transform targetLimb;
     public bool mirror;
     ConfigurableJoint cj;
 
     // Start is called before the first frame update
     void Start()
     {
         cj = GetComponent<ConfigurableJoint>();
     }
 
     // Update is called once per frame
     void Update()
     {
         if (!mirror)
         {
             cj.targetRotation = targetLimb.rotation;
         }
         else
         {
             cj.targetRotation = Quaternion.Inverse(targetLimb.rotation);
         }
     }
 }

Thanks for help!