Send a message to the FCM.

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Threading;
using System.Net;
using System.IO;
using Google.Apis.Auth.OAuth2;
using Newtonsoft.Json;

namespace FirebaseCSTest
{
    class Program
    {
        static void Main(string[] args)
        {
            string jsonFile = "service-key.json";

            ServiceAccountCredential credential;
            string token;

            using (var stream = new FileStream(jsonFile, FileMode.Open))
            {
                credential = GoogleCredential.FromStream(stream)
                    .CreateScoped(Enumerable.Repeat("https://www.googleapis.com/auth/firebase.messaging", 1))
                    .UnderlyingCredential as ServiceAccountCredential;
            }
            
            token = credential.GetAccessTokenForRequestAsync().GetAwaiter().GetResult();
            Console.WriteLine("Token: {0}", token);

            string msg = JsonConvert.SerializeObject(new
            {
                message = new
                {
                    topic = "news",
                    notification = new
                    {
                        body = "This is test. 테스트 알림입니다.\n123abc.",
                        title = "Test Alarm",
                    },
                },
            });
            var msgBytes = Encoding.UTF8.GetBytes(msg);

            string endpoint = "https://fcm.googleapis.com/v1/projects/<my-project-id>/messages:send";
            var request = WebRequest.Create(endpoint);
            request.Method = "POST";
            request.ContentType = "application/json";
            request.ContentLength = msgBytes.Length;
            request.Headers.Add(HttpRequestHeader.Authorization, $"Bearer {token}");

            using (var stream = request.GetRequestStream())
            {
                stream.Write(msgBytes, 0, msgBytes.Length);
            }

            using (var res = request.GetResponse())
            {
                Console.WriteLine(((HttpWebResponse)res).StatusDescription);
                using (var stream = new StreamReader(res.GetResponseStream()))
                {
                    Console.WriteLine(stream.ReadToEnd());
                }
            }

            Console.ReadKey();
        }
    }
}