Uses the HttpClient library for .NET

scraping.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net.Http;
using System.IO;
using System.Net;
using System.Net.Http.Headers;

namespace Avenet.Utils
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine(GetFile());
        }

        private static HttpClient CreateHttpClient(string server)
        {
            var cookies = new CookieContainer();

            var proxy = new WebProxy("proxy_url:proxy_port", false, new string[] { });

            var handler = new HttpClientHandler
            {
                CookieContainer = cookies,
                UseCookies = true,
                UseDefaultCredentials = false,
                Proxy = proxy,
                UseProxy = true,
            };

            var client = new HttpClient()
            {
                BaseAddress = new Uri(server),
            };

            var username = "username";
            var password = "password";
            var encoded = System.Convert.ToBase64String(System.Text.Encoding.UTF8.GetBytes(username + ":" + password));

            client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
            client.DefaultRequestHeaders.Add("ContentType", "application/xml");
            client.DefaultRequestHeaders.Add("Authorization", "Basic " + encoded);

            return client;
        }

        static string GetFile()
        {
            var client = CreateHttpClient("target_site");

            WebRequest.DefaultWebProxy.Credentials = CredentialCache.DefaultNetworkCredentials;

            var requestContent = new StringContent("content");

            var postRequest = client.PostAsync(
                "",
                requestContent);

            HttpResponseMessage response = postRequest.Result;
            var responseContent = response.Content;

            if (response.StatusCode == HttpStatusCode.OK)
            {
                var result = client.GetStringAsync("target-site/full_path");
                var fileString = result.Result;
                return fileString;
            }

            return null;
        }
    }
}