IHostBuilder does not contain a definition for ConfigureWebHostDefaults

Along with using Microsoft.Extensions.Hosting, ConfigureWebHostDefaults also require using Microsoft.AspNetCore.Hosting; as follows:

using Microsoft.AspNetCore.Hosting; //<-- Here it is
using Microsoft.Extensions.Hosting;

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

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

Moreover it look like your project is a Console Application. That is the problem. ConfigureWebHostDefaults is for Web Application only. So you can convert your Console Application into Web Applicaton by replacing Sdk="Microsoft.NET.Sdk" with Sdk="Microsoft.NET.Sdk.Web" in your .csproj file as follows:

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>netcoreapp3.0</TargetFramework>
    <AspNetCoreHostingModel>InProcess</AspNetCoreHostingModel>
  </PropertyGroup>

</Project>