How to move a specific texture from a shader?

From this properties of a Shader:

 _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
 
 _AlphaMap ("Additional Alpha Map (Greyscale)", 2D) = "white" {}

I want to move the _AlphaMap so I can achieve a similar mask effect. I already tried something like:

 renderer.material.SetTextureOffset ("_AlphaMap", new Vector2(offset,offset));

with no effect.

Can someone help me?