Server Admin SDK do Firebase,

server.js

 
/*
 Snippet 
 Admin SDK do Firebase
 Firebase features such as Database, Storage, and Auth by the unified Admin SDK.
 
 Go https://console.firebase.google.com/project/meu-instalador-teste/settings/serviceaccounts/adminsdk
 Download your private key 
 
 npm install --save firebase-admin
 see more https://www.npmjs.com/package/firebase-admin

*/
var admin = require("firebase-admin");

var serviceAccount = require("./private_key.json");

admin.initializeApp({
  credential: admin.credential.cert(serviceAccount),
  databaseURL: "databaseURL"
});

// This registration token comes from the client FCM SDKs.
var registrationToken = "token to send push";

// See the "Defining the message payload" section above for details
// on how to define a message payload.
var payload = {

  data: {
    
    title: "Titulo",
    body: "mensagem",   
    sound: "sound.mp3", 
    
    //this data go inside additionalData
    Custom:"string",
    OtherCustom:"string" 
    
  },

};

// Set the message as high priority and have it expire after 24 hours.
var options = {
  priority: "high",
  timeToLive: 60 * 60 * 24
};

// Send a message to the device corresponding to the provided
// registration token with the provided options.
admin.messaging().sendToDevice(registrationToken, payload, options)
  .then(function(response) {
    console.log("Successfully sent message:", response);
  })
  .catch(function(error) {
    console.log("Error sending message:", error);
  });