How to use a Proxy with HttpClient in C# .Net

using HttpClient

https://stackoverflow.com/questions/29856543/httpclient-and-using-proxy-constantly-getting-407

// First create a proxy object
var proxy = new WebProxy
{
    Address = new Uri($"http://{proxyHost}:{proxyPort}"),
    BypassProxyOnLocal = false,
    UseDefaultCredentials = false,

    // *** These creds are given to the proxy server, not the web server ***
    Credentials = new NetworkCredential(
        userName: proxyUserName,
        password: proxyPassword)
};

// Now create a client handler which uses that proxy
var httpClientHandler = new HttpClientHandler
{
    Proxy = proxy,
};

// Omit this part if you don't need to authenticate with the web server:
if (needServerAuthentication)
{
    httpClientHandler.PreAuthenticate = true;
    httpClientHandler.UseDefaultCredentials = false;

    // *** These creds are given to the web server, not the proxy server ***
    httpClientHandler.Credentials = new NetworkCredential(
        userName: serverUserName,
        password: serverPassword);
}

// Finally, create the HTTP client object
var client = new HttpClient(handler: httpClientHandler, disposeHandler: true);

using GlobalProxySelection  .Net 8.x

using System;
using System.Net;
using System.IO;
namespace Examples.Http
{
    public class TestGlobalProxySelection
    {
        public static void Main()
        {
            // Create a request for the Web page at www.contoso.com.
            WebRequest request = WebRequest.Create("http://www.contoso.com");
            // This application doesn't want the proxy to be used so it sets
            // the global proxy to an empty proxy.
            IWebProxy myProxy = GlobalProxySelection.GetEmptyWebProxy();
            GlobalProxySelection.Select = myProxy;
            // Get the response.
            WebResponse response = request.GetResponse();
            // Display the response to the console.
            Stream stream = response.GetResponseStream();
            StreamReader reader = new StreamReader(stream);
            Console.WriteLine(reader.ReadToEnd());
            // Clean up.
            reader.Close();
            stream.Close();
            response.Close();
        }
    }
}

https://learn.microsoft.com/en-us/dotnet/api/system.net.globalproxyselection.select?view=net-8.0

step by step

https://www.scrapingbee.com/blog/csharp-httpclient-proxy/