<Target Name="CopyConfig" AfterTargets="AfterBuild">
    <Copy SourceFiles="..\MyWebApp\appsettings.json" DestinationFolder="$(OutDir)" />
</Target>

  <ItemGroup>
    <None Update="..\MyWebApp\appsettings.json">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    </None>
  </ItemGroup>

var services = new ServiceCollection();
var configuration = new Microsoft.Extensions.Configuration.ConfigurationBuilder()
   .AddJsonFile("appsettings.json", reloadOnChange: true, optional: false)
   .AddInMemoryCollection(new[]
     {
       new KeyValuePair<string,string>("UseInMemoryDatabase", "true"),
     })
   .Build();
services.AddSingleton<IConfigurationRoot>(provider => configuration);