WebPlayer WWW Authorization always prompts for credentials

Hello, when I upload my game to my web site (using Windows/IIS) and then connect to the web site via the browser it prompts me for the web service credentials.

In my code I have filled in the credentials in the header and tried every thing I can think of but nothing makes this dialog go away except if I type in the user name and password in the browser which customers wouldn't (and shouldn't) know the credentials for.

Here is my code:

 string url = Consts.MyAPIUrl;

 public IEnumerator TestWebService()
 {
     WWW www;
     byte[] toEncodeAsBytes = System.Text.Encoding.UTF8.GetBytes(UserName + ":" + UserPassword);
     string base64 = System.Convert.ToBase64String(toEncodeAsBytes); 
 
     WWWForm form = new WWWForm(); 
     var headers = form.headers;
     byte[] rawData = form.data;
     headers["Authorization"] = "Basic " + System.Convert.ToBase64String(System.Text.Encoding.ASCII.GetBytes(UserName + ":" + UserPassword));
     www = new WWW(url, rawData, headers);
     yield return www;            
 
     if (null != www) 
     {
         List<MyData> data= null;
         data = (List<MyData>)LitJson.JsonMapper.ToObject<List<MyData>> (www.text);
     }
 }