Firebase Custom Messages iOS

In order to send a push notification to a device, You need to have a script (or a piece of code) ideally hosted on a server that would send a push notification on your behalf.
There you can customize the message and even play an audio upon receiving a notification.
Here is a code snippet in java that can be used to send a push notification to a device (or a group of devices).

private Map sendPush(String to, String from, String title, String message,
            String sound) throws IOException {
    sound = (sound != null) ? sound : "default";  // set default audio file name
    // Creating the URL and connection
    URL url = new URL(FCM_URL); // your firebase URL
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setDoOutput(true);
    conn.setRequestMethod("POST");
    conn.setRequestProperty("Content-Type", "application/json");
    conn.setRequestProperty("Authorization", "key=" + FCM_KEY); // the firebase project key 

    conn.setDoOutput(true);

    // set the notification body
    Map<String, String> notificationBody = new HashMap();
    notificationBody.put("title", title); // notification title
    notificationBody.put("body", message); // notification message
    notificationBody.put("sound", sound);
    notificationBody.put("badge", "1");

    Map<String, String> dataBody = new HashMap();
    dataBody.put("sender", from); // sender id

    Map<String, Object> pushBody = new HashMap();
    pushBody.put("notification", notificationBody);
    pushBody.put("data", dataBody);
    pushBody.put("to", to); // receiver(s) id
    pushBody.put("priority", "high");

    // convert your dictionary to json string using Google Gson library (similar to JsonSerialization class in swift)
    String input = new Gson().toJson(pushBody);

    // write input bytes in request body
    try (OutputStream os = conn.getOutputStream()) {
        os.write(input.getBytes());
        os.flush();
    }

    StringBuilder responseString;
    Reader reader = new InputStreamReader(conn.getInputStream()); // send request and receive response 

    // parse response
    try (BufferedReader in = new BufferedReader(reader)) {
        String inputLine;
        responseString = new StringBuilder();
        while ((inputLine = in.readLine()) != null) {
            responseString.append(inputLine);
        }
    }

    // using Google Gson to convert json string into Map (similar to JsonSerialization class in swift)
    Map<String, Object> responseObject = new Gson().fromJson(responseString.toString(),
                Map.class);

    return responseObject;
}

Since this is a java code, so i have hosted it in a java application deployed on Apache Tomcat Server.

You can find several similar implementations in various languages like php or node.js etc.

Hope this helps