How to manually send a Message to an instance of FirebaseMessagingService?

First of all FCM is used for push Notification. Not for internal communication.

<service android:name=".fcms.MyFirebaseMessagingService"
            android:stopWithTask="false">
            <intent-filter>
                <action android:name="com.google.firebase.MESSAGING_EVENT" />
            </intent-filter>
        </service>

along with

        <meta-data
            android:name="com.google.firebase.messaging.default_notification_color"
            android:resource="@color/colorAccent" />

        <meta-data
            android:name="com.google.firebase.messaging.default_notification_channel_id"
            android:value="@string/default_notification_channel_id" />

MyFirebaseMessagingService.kt

class MyFirebaseMessagingService : FirebaseMessagingService() {

    val TAG :String = "MessagingService"

    override fun onNewToken(str: String?) {
        super.onNewToken(str)

        //send token to server... if user is logged In...
    }

    override fun onMessageReceived(remoteMessage: RemoteMessage?) {
        super.onMessageReceived(remoteMessage)

        /*Map<String, String> params = remoteMessage.getData();
        JSONObject object = new JSONObject(params);
        Log.e("JSON_OBJECT", object.toString());*/

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

            /*Log.e(TAG, "collapsekey: " + remoteMessage.getCollapseKey());
            Log.e(TAG, "from: " + remoteMessage.getFrom());
            Log.e(TAG, "message id: " + remoteMessage.getMessageId());
            Log.e(TAG, "message type:: " + remoteMessage.getMessageType());
            Log.e(TAG, "to: " + remoteMessage.getTo());
            Log.e(TAG, "send time: " + remoteMessage.getSentTime());
            Log.e(TAG, "ttl: " + remoteMessage.getTtl());
            Log.e(TAG, "title: " + remoteMessage.getNotification().getTitle());
            Log.e(TAG, "body: " + remoteMessage.getNotification().getBody());
            Log.e(TAG, "click action: " + remoteMessage.getNotification().getClickAction());
            Log.e(TAG, "color: " + remoteMessage.getNotification().getColor());
            Log.e(TAG, "icon: " + remoteMessage.getNotification().getIcon());*/
        }

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

            val notificationManager = getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager
            val NOTIFICATION_CHANNEL_ID = "broadpeak.notification"

            if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O) {

                val name = "my_channel"
                val Description = "This is my channel"
                val importance = NotificationManager.IMPORTANCE_HIGH
                val mChannel = NotificationChannel(NOTIFICATION_CHANNEL_ID, name, importance)
                mChannel.description = Description
                mChannel.enableLights(true)
                mChannel.lightColor = Color.RED
                mChannel.enableVibration(true)
                mChannel.vibrationPattern = longArrayOf(100, 200, 300, 400, 500, 400, 300, 200, 400)
                mChannel.setShowBadge(false)
                notificationManager.createNotificationChannel(mChannel)
            }

            val notificationBuilder = NotificationCompat.Builder(this, NOTIFICATION_CHANNEL_ID)

            val tagID = remoteMessage?.data!!.get("tagID").toString()
            val tagName = remoteMessage?.data!!.get("tagName").toString()

            val intent = Intent(this, McqActivity::class.java)
            intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);// | Intent.FLAG_ACTIVITY_NEW_TASK)
            intent.putExtra("keyTagBO",SubjectBO(tagID,tagName))
            val resultIntent = PendingIntent.getActivity(this, 0, intent,
                    PendingIntent.FLAG_UPDATE_CURRENT)

            val startIntent  = PendingIntent.getService(this,0,
                    Intent(this,BackgroundService::class.java).setAction("START"),PendingIntent.FLAG_UPDATE_CURRENT)

            val stopIntent  = PendingIntent.getService(this,0,
                    Intent(this,BackgroundService::class.java).setAction("STOP"),PendingIntent.FLAG_UPDATE_CURRENT)

            notificationBuilder.setAutoCancel(true)
                    .setDefaults(Notification.DEFAULT_ALL)
                    .setWhen(System.currentTimeMillis())
                    .setSmallIcon(R.drawable.round_green_layout)
                    .setOngoing(true)//If you want not to remove it on Swipe
                    .setContentTitle(remoteMessage.notification!!.title)
                    .setSound(RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION))
                    .setVibrate(longArrayOf(0, 1000, 500, 1000))
                    .setContentText(remoteMessage.notification!!.body)
                    .setContentIntent(resultIntent)
                    .addAction(R.mipmap.ic_launcher,"Start Action",startIntent)
                    .addAction(R.mipmap.ic_launcher,"Stop Action",stopIntent)
                    .setContentInfo("Info")

            notificationManager.notify(Random().nextInt(), notificationBuilder.build())
        }
    }

Go to firebase console and select Cloud Messaging and create a notification .. and pass key value pair if you like

Try this...

RemoteMessage remoteMessage = new RemoteMessage.Builder(<ANY_STRING>) 
                                                .addData(<YOUR_KEY_HERE>, <VALUE>)
                                                .build();
PushReceiver pushReceiver = new PushReceiver(getContext());
pushReceiver.onMessageReceived(remoteMessage);