C# FCM for Android Push Notification

I am trying to use FCM for pushing notifications to Android device but it seems that I am missing something. These are steps that I've followed

Create new project in https://console.firebase.google.com
From Settings -> Cloud Messaging tab, I got Server key and Sender Id

I used this code

    var regId = "device id";
    string SERVER_API_KEY = "Firebase Server key";
    var SENDER_ID = "Sender ID";
    var value = "Test Notification";
    WebRequest tRequest;
    tRequest = WebRequest.Create("https://fcm.googleapis.com/fcm/send");
    tRequest.Method = "post";
    tRequest.ContentType = " application/x-www-form-urlencoded;charset=UTF-8";
    tRequest.Headers.Add(string.Format("Authorization: key={0}", SERVER_API_KEY));

    tRequest.Headers.Add(string.Format("Sender: id={0}", SENDER_ID));

    string postData = "collapse_key=score_update&time_to_live=108&delay_while_idle=1&data.message=" + value + "&data.time=" + System.DateTime.Now.ToString() + "&registration_id=" + regId + "";
    Console.WriteLine(postData);
    byte[] byteArray = Encoding.UTF8.GetBytes(postData);
    tRequest.ContentLength = byteArray.Length;

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

    WebResponse tResponse = tRequest.GetResponse();

    dataStream = tResponse.GetResponseStream();

    StreamReader tReader = new StreamReader(dataStream);

    string sResponseFromServer = tReader.ReadToEnd();


    tReader.Close();
    dataStream.Close();
    tResponse.Close();

I always get Mismatch Sender Id error. Am I missing any steps? or do I have to do something on mobile app?

I found the error. As a back-end developer I worked on C# part only. There is one more step to be done from mobile application side. Mobile app should allow receiving messages from list of Sender Ids so the one that I use in my C# code must be allowed from mobile app side. Once I did that it worked