How to use GrabPass in DrawProcedural?

Unity Engine
1

I use Surface Shader to render geometry generated with Compute Shader.
I draw geometry from vertex buffer using DrawProcedural.
I wanna make surface semi-transparent. I read I have to use GrabPass to get texture behind my material. It works on static meshes but doesnt work on my generative mesh.

How can I use GrabPass or how can I get texture behind my material when and I use DrawProcedural?

I have this weird error dont know what it means

Parse error: syntax error, unexpected TOK_PASS, expecting TOK_SETTEXTURE or '}'
2

Given this is a syntax error, it would be helpful to see your shader code.