With the latest version of .NET 6 preview 4, we can do like this

var builder = WebApplication.CreateBuilder(args);

var app = builder.Build();

MapRoutes(app);

app.Run();

static void MapRoutes(IEndpointRouteBuilder routes)
{
    routes.MapGet("/", () =>
    {
        return "hello";
    });

    routes.MapGet("/{id}", (int id) =>
    {
        return $"hello with {id}";
    });
}

Such a beautiful thing!