Once you're setup with the firebase in your application, you can send push notification through any source, I am using Nodejs.
Step 1 : Install FCM in node project Go here https://www.npmjs.com/package/fcm or add
Step 2 : Include fcm in your controller
var FCM = require('fcm-push');
Step 3 : Configure fcm in your project
var fcm = new FCM(serverKey);
Step 4 : Trigger / Push the push notification
Now you're done !