How to get user0 channel data in shader

I need to obtain the color data of the user0 channel created by the artist in the shader file. If I can obtain this color data, I can perform some calculations on it and finally add it to the rendering result.

Project situation: The artist wants to draw special strokes on the world space normal, that can produce special lighting effects. I can achieve the above effect by baking the texture and then reimporting it, but the efficiency is relatively low. The artist hopes to see the real-time lighting effect when drawing.