Hi John, can you please try with the below code.
var httpRequest = (HttpWebRequest)WebRequest.Create(@"https://yoursite/fileToDownload.ext");
httpRequest.Credentials = new NetworkCredential("username", "password", "domainname");
var httpResponse = (HttpWebResponse)httpRequest.GetResponse();
var downloadStrm = httpResponse.GetResponseStream();
var donloadStrm = new StreamReader(downloadStrm);
downloadStrm.Close();