Download from URL on a proxy Server

The code below works on my local machine.  However to connect to external websites on the production server, I have to set the hostname to ftpProxy.

Example:

On the old server I would use these credentials.

set HOSTNAME=ftp.site.com
set USERID=user
set PWD=yyy

On the new server I would use these.

set HOSTNAME=ftpproxy
set USERID=user@ftp.site.com
set PWD=yyy

How do I get this to work on the production server?  I'm using a C# windows form program.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.IO;
namespace WebDownloader
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            try
            {
                WebClient client = new WebClient();
                string Webpage = client.DownloadString("https://www.Site.com/xmlfeed/xml_prices_full.asp?code=99201&user=xxxml&pass=yy");
                using (StreamWriter sw = new StreamWriter("O:\\Data_Download\\Supplier\\SupplierFile.txt"))
                {
                    sw.Write(Webpage);
                }
                Webpage = client.DownloadString("https://www.Site.com/xmlfeed/xml_prices_full.asp?code=99221&user=xxxml&pass=yy");
                using (StreamWriter sw = new StreamWriter("O:\\Data_Download\\Supplier\\SupplierFileMBIO.txt"))
                {
                    sw.Write(Webpage);
                }

                Environment.Exit(0);
            }
            catch (Exception ex)
            {
                using (StreamWriter sw2 = new StreamWriter("O:\\Loaders\\Global\\Supplier\\Commodities\\Price\\Errors\\DownloadError.txt"))
                {
                    sw2.Write(ex.Message);
                    
                }
                Environment.Exit(1);
            }
        }
    }
}

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