You can try the below code:-

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Net;
using System.Text;
using System.IO;
protected void Button1_Click(object sender, EventArgs e)
{
//Registration Id created by Android App i.e. DeviceId.  
string regId;  
regId = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";   
//API Key created in Google project  
var applicationID = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";   

//Project ID created in Google project.  
 var SENDER_ID = "xxxxxxxxxxxx";   
 var varMessage = Hello! How are you?;  
 WebRequest tRequest;  
 tRequest = WebRequest.Create("https://android.googleapis.com/gcm/send");  
 tRequest.Method = "post";  
 tRequest.ContentType = " application/x-www-form-urlencoded;charset=UTF-8";  
 tRequest.Headers.Add(string.Format("Authorization: key={0}", applicationID));  

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

 string postDataToServer = "collapse_key=score_update&time_to_live=108&delay_while_idle=1&data.message="  
                    + varMessage + "&data.time=" + System.DateTime.Now.ToString() + "istration_id=" + regId + "";  


Console.WriteLine(postDataToServer);  
Byte[] byteArray = Encoding.UTF8.GetBytes(postDataToServer);  
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();  
}

For information visit this https://developers.google.com/cloud-messaging/