Program.cs

using System;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Hosting;

namespace ASPDotNetCore3Crawl
{
    public class Program
    {
        public static void Main(string[] args)
        {
            CreateHostBuilder(args).Build().Run();
        }

        public static IHostBuilder CreateHostBuilder(string[] args) 
        {
            string port=Environment.GetEnvironmentVariable("PORT");
            if (port == null || port=="") port="5000";

            Console.WriteLine("Port:"+port);
            return Host.CreateDefaultBuilder(args)
                    .ConfigureWebHostDefaults(webBuilder =>
                    {
                        webBuilder.UseStartup<Startup>()
                            .UseUrls("http://*:"+port);
                    });
        }
    }
}