How can I enable GPU instancing on terrain details?

I have tried to follow this manual entry: https://docs.unity3d.com/Manual/GPUInstancing.html

But there isn't a user accessible terrain detail material available to check the "enable GPU instancing checkbox". I also tried to edit the "Hidden/TerrainEngine/Details/Vertexlit" to add GPU instancing directives based on this example from the manual:

 Shader "SimplestInstancedShader"
 {
     Properties
     {
         _Color ("Color", Color) = (1, 1, 1, 1)
     }

     SubShader
     {
         Tags { "RenderType"="Opaque" }
         LOD 100

         Pass
         {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #pragma multi_compile_instancing
             #include "UnityCG.cginc"

             struct appdata
             {
                 float4 vertex : POSITION;
                 UNITY_VERTEX_INPUT_INSTANCE_ID
             };

             struct v2f
             {
                 float4 vertex : SV_POSITION;
                 UNITY_VERTEX_INPUT_INSTANCE_ID // necessary only if you want to access instanced properties in fragment Shader.
             };

             UNITY_INSTANCING_BUFFER_START(Props)
                 UNITY_DEFINE_INSTANCED_PROP(float4, _Color)
             UNITY_INSTANCING_BUFFER_END(Props)
         
             v2f vert(appdata v)
             {
                 v2f o;

                 UNITY_SETUP_INSTANCE_ID(v);
                 UNITY_TRANSFER_INSTANCE_ID(v, o); // necessary only if you want to access instanced properties in the fragment Shader.

                 o.vertex = UnityObjectToClipPos(v.vertex);
                 return o;
             }
         
             fixed4 frag(v2f i) : SV_Target
             {
                 UNITY_SETUP_INSTANCE_ID(i); // necessary only if any instanced properties are going to be accessed in the fragment Shader.
                 return UNITY_ACCESS_INSTANCED_PROP(Props, _Color);
             }
             ENDCG
         }
     }
 }

But that doesn't seem to work. I have googled extensively on instancing terrain detail, but only a commercial asset surfaces. I'm surprised that the default terrain system doesn't promote instancing details, as they are ideal candidates for this optimization.

I have a lot of grass on my terrain, any help is greatly appreciated.