Power BI Bearer token REST API throws error The remote server returned an error: (400) Bad Request.

I am trying to refresh PBI dataset programmatically and to do so I am trying to get oath beaer token using the code below. The code works fine with a console app. But when I embed the code into a WCF web service and deploy in a web app then it throws 400) Bad Request error. The reason I had to pass user id and password because the WCF web service will run in backend anf will not have any GUI interaction for user authentication. 

resource -> https://analysis.windows.net/powerbi/api

authority -> https://login.microsoftonline.com/*****tenant Id****/oauth2/token

cleint id and ApplicationSecret has been collected from PBI app registration.

StringBuilder body = new StringBuilder();
            body.Append("resource=" + HttpUtility.UrlEncode(resourceUri));
            body.Append("&client_id=" + HttpUtility.UrlEncode(ApplicationID));
            body.Append("&grant_type=" + HttpUtility.UrlEncode("password"));
            body.Append("&username=" + HttpUtility.UrlEncode(userid));
            body.Append("&password=" + HttpUtility.UrlEncode(password));
            body.Append("&client_secret=" + HttpUtility.UrlEncode(ApplicationSecret));
            
            using (WebClient web = new WebClient())
            {                
                web.Headers.Add("Content-Type", "application/x-www-form-urlencoded");               
                string data = web.UploadString(authority, body.ToString()); // this line throws error
                dynamic result = JsonConvert.DeserializeObject(data);
                try
                {
                    return result.access_token;
                }
                catch
                {

                }
                return null;
            }