Help! Additive Reference Pose Problem

Hello, I have a Generic avatar. I create Animation Clip, I make some animation but when I add this into second layer in Animation tab, I make both Additive but nothing happens.
I searched about it and I can't see any Additive Reference Pose checkbox..
The thing I want to reach is;
There is two animation: one is Idle other is long length joints I mean character height. They are not work same time. Additive does nothing.

 

I may be wrong, but I think that in order to see the checkbox you have to enter debug mode (go to the upper right corner of the inspector window and right click on the three line icon, here you choose debug mode). It seems you can also set the ref pose using this function: https://docs.unity3d.com/ScriptReferenc/AnimationUtility.SetAdditiveReferencePose.html
This thread seems to have every tip you may need:
https://forum.unity.com/threads/additive-animations-not-working.419382/