how to approach instanceID in shader?

SkinnedMeshRenderer didn't batched in Unity. so i will implement GPU Skinning with GPU Instancing. i try to approach instanceID(like SV_InstanceID), but they are wrapped in preprocessor macro. so i find in built in shader(version is 5.6.1f). and i found this comment.

 // basic instancing setups
 // - UNITY_VERTEX_INPUT_INSTANCE_ID     Declare instance ID field in vertex shader input / output struct.
 // - UNITY_GET_INSTANCE_ID  (Internal) Get the instance ID from input struct.

and im in frustration.. "Internal" keyword make me embrassed. i have just only one question.

Can i use "instanceID" in shader code?