how to fix "The remote server returned an error: (403) Forbidden"

This error has been asked several times across different forums and sites. I have tried nearly all the solutions but still unable to fix it. My code is as below.

public XmlDocument PlaceOrder(DeliveryModel model, BIUser theUser, Guid orderId, illunit theUnit, string speed)
    {
        string xmlToPost = ConstructXMLOrderRequest(model, theUser, orderId, theUnit, speed);
        string authParams = GenerateAuthenticationString(xmlToPost, theUnit.bl_acc);
        string rootUrl = configurationLogic.LibraryAPIUrl;
        Encoding enc = System.Text.Encoding.GetEncoding(1252);
        string requestString = rootUrl + "orders";

        HttpWebRequest apiCall = (HttpWebRequest)WebRequest.Create(requestString);

        apiCall.Headers.Add("BLDSS-API-Authentication", authParams);


        byte[] byteArray = ASCIIEncoding.UTF8.GetBytes(xmlToPost);
        apiCall.Accept = "*/*";
        apiCall.KeepAlive = false;
        apiCall.AllowAutoRedirect = false;
        apiCall.Method = "POST";
        apiCall.ContentType = "text/xml";
        apiCall.ContentLength = byteArray.Length;
        apiCall.UserAgent = "Code Sample Web Client";

        Stream dataStream = apiCall.GetRequestStream();
        dataStream.Write(byteArray, 0, byteArray.Length);
        dataStream.Close();

        HttpWebResponse webresponse;
        webresponse = (HttpWebResponse)apiCall.GetResponse();

        StreamReader loResponseStream = new StreamReader(webresponse.GetResponseStream(), enc);
        string response = loResponseStream.ReadToEnd();

        XmlDocument toReturn = new XmlDocument();
        toReturn.LoadXml(response);

        return toReturn;
    }

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thanks