Startup

	public class Startup
	{
		public IConfiguration Configuration { get; }

		public Startup()
		{
			var builder = new ConfigurationBuilder()
				.AddEnvironmentVariables();
			Configuration = builder.Build();
		}

		public void ConfigureServices(IServiceCollection services)
		{
			services.Configure<MyPoco>(options =>
			{
				options.FooBar = new Uri(Configuration["Foo"]);                             <======= How do I get this from the configuration?
			});
		}
}

Xunit fixture

var builder = new WebHostBuilder()
.UseSetting("Foo", "Bar")
.UseStartup<Startup>();

_server = new TestServer(builder);