Instantiating with the desired rotation

So i want to instantiate an object onto a specific point (not difficult) but with a specific rotation changed. i want the y axis to be + 90 of what the object it is instantiating on is. For example if i am instantiating on a object with the y rotation of 90. i want to to instantiate with the y rotation of 180. i want X,Z to be the same rotations however. How would i get around doing this? i initially thought it would be quite simple but i just cant get it (even though it probably is simple). Thanks in advance for helping me out.

GameObject.transform.rotation.eulerAngles.y + 90 makes it work