C#

Form1:

public partial class Form1 : Form
{


    public Form1()
    {
        InitializeComponent();

    }

    private void buttonOpenForm2 _Click(object sender, EventArgs e)
    {
        Form2 frm2 = new Form2();
        frm2.Show();
    }

}


Form2:

public partial class Form2 : Form
{
    public Form2()
    {
        InitializeComponent();
    }

    private void buttonOpenForm3 _Click(object sender, EventArgs e)
    {
        Form3 frm3 = new Form3();
        frm3.Show();
    }
}



Form3:

  public partial class Form3 : Form
  {
    public Form3()
    {
        InitializeComponent();
    }

    private void buttonAddRow _Click(object sender, EventArgs e)
    {
        //What to write here to insert the 3 textboxes values into DataGridView?

    }
}

public class GcmSender
    {
        private const string GcmUri = "https://android.googleapis.com/gcm/send";

        #region Properties

        public string DeviceToken { get; set; }
        public string ApiKey { get; set; }
        public string ResultJson { get; private set; }

        #endregion

        HttpWebRequest _gcmRequest;
        HttpWebResponse _gcmResponse;

        public GcmSender()
        {
            // do nothing
        }

        public GcmSender(string deviceToken, string apiKey)
        {
            DeviceToken = deviceToken;
            ApiKey = apiKey;
        }

        public string Send(string message)
        {
            // Escape condition
            if (DeviceToken == null || ApiKey == null)
            {
                return "[ERROR] Device Token or API Key has not been set";
            }

            InitGcmClient();
            PostPayload(message);

            try
            {
                _gcmResponse = _gcmRequest.GetResponse() as HttpWebResponse;
            }
            catch (WebException we)
            {
                return "[ERROR] There is a problem within processing GCM message \n" + we.Message;
            }
            ResultJson = ReadResponse(_gcmResponse);

            return ResultJson;
        }

        private string ReadResponse(HttpWebResponse response)
        {
            StreamReader responseReader = new StreamReader(response.GetResponseStream());
            return responseReader.ReadToEnd();
        }

        private void InitGcmClient()
        {
            _gcmRequest = WebRequest.Create(GcmUri) as HttpWebRequest;
            if (_gcmRequest != null)
            {
                _gcmRequest.ContentType = "application/json";
                _gcmRequest.UserAgent = "Android GCM Message Sender Client 1.0";
                _gcmRequest.Method = "POST";

                // Credential info
                _gcmRequest.Headers.Add("Authorization", "key=" + ApiKey);
            }
        }

        private void PostPayload(string message)
        {
            // Ready
            string payloadString = AssembleJSONPayload(DeviceToken, message);
            byte[] payloadByte = Encoding.UTF8.GetBytes(payloadString);
            _gcmRequest.ContentLength = payloadByte.Length;

            // Go
            using (Stream payloadStream = _gcmRequest.GetRequestStream())
            {
                payloadStream.Write(payloadByte, 0, payloadByte.Length);
                payloadStream.Close();
            }
        }

        private string AssembleJSONPayload(string gcmDeviceToken, string gcmBody)
        {
            string payloadFormatJSON =
                "{{" +
                    "\"registration_ids\" : [\"{0}\"]," +
                    "\"data\" : {{" +
                        " {1} " +
                    "}}" +
                "}}";

            string payload = string.Format(payloadFormatJSON, gcmDeviceToken, gcmBody);
            Debug.WriteLine("payload : " + payload);

            return payload;
        }
    }

_gcmResponse = _gcmRequest.GetResponse() as HttpWebResponse;

The remote server returned an error: (400) Bad Request.