var oAuthConsumerKey = "";
 var oAuthConsumerSecret = "";
 var oAuthUrl ="https://quickbooks.api.intuit.com/v3/company/<companyid>/invoice?minorversion=4";
    // Do the Authenticate
 var authHeaderFormat = "Basic {0}";
 var authHeader = string.Format(authHeaderFormat,Convert.ToBase64String(Encoding.UTF8.GetBytes(Uri.EscapeDataString(oAuthConsumerKey) + ":"+Uri.EscapeDataString((oAuthConsumerSecret))) ));
    string text = System.IO.File.ReadAllText("text file");
    //  var postBody = "grant_type=client_credentials";
    HttpWebRequest authRequest = (HttpWebRequest)WebRequest.Create(oAuthUrl);
    authRequest.Headers.Add("Authorization", authHeader);
    authRequest.Method = "POST";
    authRequest.ContentType = "application/x-www-form-urlencoded";
    authRequest.UserAgent = "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 7.1; Trident/5.0)";
    authRequest.Accept = "/";
    authRequest.UseDefaultCredentials = true;
    authRequest.Proxy.Credentials = System.Net.CredentialCache.DefaultCredentials;
    authRequest.AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate;
   using (Stream stream = authRequest.GetRequestStream())
    {
        byte[] content = ASCIIEncoding.ASCII.GetBytes(text);
        stream.Write(content, 0, content.Length);
    }
   authRequest.Headers.Add("Accept-Encoding", "gzip");
   WebResponse authResponse = authRequest.GetResponse();