Using old style app host-builder with `Startup.cs`

Program.cs

public class Program
{
    public static void Main(string[] args)
        => CreateHostBuilder(args).Build().Run();

    public static IHostBuilder CreateHostBuilder(string[] args)
        => Host.CreateDefaultBuilder(args)
               .ConfigureWebHostDefaults(webBuilder => webBuilder.UseStartup<Startup>());
}

Startup.cs

public class Startup
{
    public IConfiguration Configuration { get; }

    public Startup(IConfiguration configuration) { Configuration = configuration; }

    public void ConfigureServices(IServiceCollection services)
    {
        services.AddFastEndpoints()
                .SwaggerDocument();
    }

    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        app.UseRouting()
           .UseEndpoints(x => x.MapFastEndpoints())
           .UseSwaggerGen();
    }
}