flip shader UV reading on ios

Hi,

I use a plugin to paint on a plane mesh with a mesh collider using touch position. It's working fine on Android, however on iOS there is a flip/mirrored problem. When I touch on the top of the mesh, paint is applied on the bottom.

I read about a shader issue depending on the plateform here :

https://docs.unity3d.com/Manual/SL-PlatformDifferences.html

where there is an exemple :

 // Flip sampling of the Texture: 
 // The main Texture
 // texel size will have negative Y).
 
 #if UNITY_UV_STARTS_AT_TOP
 if (_MainTex_TexelSize.y < 0)
         uv.y = 1-uv.y;
 #endif

it looks like my problem solution, but I dont know where to put this code in the shader to solve the problem...

Any idea about that or another workaround ?

Thank you very much for your help.