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();