To send a notification, you can use the web interface of the service or you can use the features that the service provides if you want to send a message programmatically.
The following is a cloud code example for the Parse Server (Back4App). It sends a message to all devices that are listening to a particular channel. You can send a push notification to all users or you can set up channels for customer segmentation. You can set the number of the badge on the icon (iOS only), the title, and the message:
Parse.Push.send({ channels: "channel or channels", data: { title: "title", sound: 'default', badge: 2, alert: "message", extraParam: "something" } }, { success: function () { response.success("ok"); }, error: function (error) { response.success("nok: " + error); }...