You can add the cubemap to the material at runtime (see Can't seem to change cubeMap on Material by Code)
Or add it the scene at runtime and don’t render the skybox layer (see Changing cubemap settings on the fly (via code) not producing desired results)
Or add it to the scene in the Editor and don’t render the skybox layer which is what I did here: How to use realtime environment as the cubemap? - #3 by yaustar