Implement texture masking into simple existing shader

Hello, I have this shader, and I need implement _AlphaTex (black and white texture) as transparent mask for _MainTex.

 Shader "Custom/Custom Transparent" {
     Properties {
         _MainTex ("Base (RGB)", 2D) = "white" {}
         _AlphaTex ("Alpha mask", 2D) = "white" {} // I need implement this texture as Transparent mask for _MainText
     }
     SubShader {
     
         LOD 100
     
         Tags
         {
             "Queue" = "Transparent"
             "IgnoreProjector" = "True"
             "RenderType" = "Transparent"
         }
           
           Pass
           {
               Cull Off
             Lighting Off
             ZWrite Off
             Fog { Mode Off }
             Offset -1, -1
             ColorMask RGB
             AlphaTest Greater .01
             Blend SrcAlpha OneMinusSrcAlpha
             ColorMaterial AmbientAndDiffuse 
           
              SetTexture [_MainTex] {Combine Texture * Primary}
           }
         
     } 
 }