Send push notification with FCM direct from an activity on android

Good evening! I am having difficulty adding the functionality of sending notifications to the application users, by the Firebase console these notifications are already sent and received, what I wanted now is from the app itself the user can send a notification in which he can write the message to be sent, but I could not send it through the app, even following the firebase documentation.

This is the class I copied directly from the github of firebase, but I have no idea how to implement it

public class MyFirebaseMessagingService extends FirebaseMessagingService {
private final static String TAG = "FCM";

//Recebendo a mensagem
@Override
public void onMessageReceived(RemoteMessage remoteMessage) {
    Log.d(TAG, "From: " + remoteMessage.getFrom());

    // Check if message contains a data payload.
    if (remoteMessage.getData().size() > 0) {
        Log.d(TAG, "Message data payload: " + remoteMessage.getData());

        if (/* Check if data needs to be processed by long running job */ true) {
            // For long-running tasks (10 seconds or more) use Firebase Job Dispatcher.
            scheduleJob();
        } else {
            // Handle message within 10 seconds
            handleNow();
        }

    }

    // Check if message contains a notification payload.
    if (remoteMessage.getNotification() != null) {
        Log.d(TAG, "Message Notification Body: " + remoteMessage.getNotification().getBody());
    }

    super.onMessageReceived(remoteMessage);
}

/**
 * Schedule a job using FirebaseJobDispatcher.
 */
private void scheduleJob() {
    // [START dispatch_job]
    FirebaseJobDispatcher dispatcher = new FirebaseJobDispatcher(new GooglePlayDriver(this));
    Job myJob = dispatcher.newJobBuilder()
            .setService(MyJobService.class)
            .setTag("my-job-tag")
            .build();
    dispatcher.schedule(myJob);
    // [END dispatch_job]
}

/**
 * Handle time allotted to BroadcastReceivers.
 */
private void handleNow() {
    Log.d(TAG, "Short lived task is done.");
}

/**
 * Create and show a simple notification containing the received FCM message.
 *
 * @param messageBody FCM message body received.
 */
private void sendNotification(String messageBody) {
    Intent intent = new Intent(this, MainActivity.class);
    intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
    PendingIntent pendingIntent = PendingIntent.getActivity(this, 0 /* Request code */, intent,
            PendingIntent.FLAG_ONE_SHOT);

    String channelId = getString(R.string.default_notification_channel_id);
    Uri defaultSoundUri= RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
    NotificationCompat.Builder notificationBuilder =
            new NotificationCompat.Builder(this, channelId)
                    .setSmallIcon(R.drawable.ic_stat_ic_notification)
                    .setContentTitle("FCM Message")
                    .setContentText(messageBody)
                    .setAutoCancel(true)
                    .setSound(defaultSoundUri)
                    .setContentIntent(pendingIntent);

    NotificationManager notificationManager =
            (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);

    // Since android Oreo notification channel is needed.
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        NotificationChannel channel = new NotificationChannel(channelId,
                "Channel human readable title",
                NotificationManager.IMPORTANCE_DEFAULT);
        notificationManager.createNotificationChannel(channel);
    }

    notificationManager.notify(0 /* ID of notification */, notificationBuilder.build());
}
}

The idea is for a user to write a message in the app and when it clicks a button it will be triggered for users who have the app in the background;