How to use Camera.RenderWithShader() to modify rendered image?

Hi

Can anybody please help me understand how to work with the Camera.RenderWithShader() function?

I am rendering one of my cameras into a RenderTexture and everything works as it should, however I desperately need to "trim" the rendered image by cutting off (via alpha), lets say, 10% of pixels from left, right, top and bottom sides...

I believe I can cut off parts of the texture with the help of the following shader, however it requires a texture _TestTex to work... And I have no idea how to pass a texture to this shader :( I tried calling

Shader.SetGlobalTexture("_TestTex", myCamera.targetTexture);

and

Shader.SetGlobalTexture("_TestTex", myCamera.renderer.material.mainTexture);,

but I'm definitely doing something wrong... Please help me solve my dilemma!

Shader "Test/TestShader" { Properties { _TestTex ("Base (RGB)", 2D) = "white" {} }

 SubShader
 {
     Tags
     {
         "RenderType" = "Opaque"
         "Queue" = "Transparent"
         "IgnoreProjector" = "True"
     }

     LOD 100

     Pass
     {
         Cull Off
         Lighting Off
         ZWrite Off
         Offset -1, -1
         Fog { Mode Off }
         Blend SrcAlpha OneMinusSrcAlpha

         CGPROGRAM
         #pragma vertex vert
         #pragma fragment frag
         #pragma fragmentoption ARB_precision_hint_fastest

         #include "UnityCG.cginc"

         sampler2D _TestTex;

         struct appdata_t
         {
             fixed4 vertex : POSITION;
             fixed4 color : COLOR;
             fixed2 texcoord : TEXCOORD0;
         };

         struct v2f
         {
             fixed4 vertex : POSITION;
             fixed4 color : COLOR;
         };

         v2f vert (appdata_t v)
         {
             v2f o;
             o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
             o.color.xy = v.texcoord.xy;
             o.color.zw = o.vertex.xy;
             return o;
         }

         fixed4 frag (v2f IN) : COLOR
         {
             fixed4 result = fixed4(0, 0, 0, 0);

             if (IN.color.z < 0.1 || IN.color.z > 0.8 || IN.color.w < 0.1 || IN.color.w > 0.8)
             {
                 result.rgb = tex2D(_TestTex, IN.color.xy).rgb;
                 result.a = 1;
             }
             
             return result;
         }
         ENDCG
     }
 }

}