C# WPF Visual Studio v file read variable from custom config

I want to access in WPF .csproj some custom variable from custom config file in my Solution.

I have edited .csproj file in Notepad++. Like $(Configuration) variable, I tried to use custom variable $(NameOfCustomVariable), but no luck.

That NameOfCustomVariable is boolean. Depend on it's value I will change ProductName.Something like this pseudo code:

<ProductName>Example $(Configuration) (if $(NameOfCustomVariable) == true 'TEST' else 'PROD')</ProductName>

But I am not able to make this work. Any suggestion?