Working EFConfigProvider

public static IWebHost BuildWebHost(string[] args)
{
    var config = new ConfigurationBuilder()
        .SetBasePath(Directory.GetCurrentDirectory())
        .AddJsonFile("appsettings.json");

    return WebHost.CreateDefaultBuilder(args)
        .ConfigureAppConfiguration((context, builder) => {
            builder.AddEntityFrameworkConfig(options => options.UseSqlServer(config.GetConnectionString("DefaultConnection")));
            });
        })
        .UseStartup<Startup>()
        .Build();
}