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I’m trying to get Firebase Cloud Messaging to send a message to a specific user but I’m not getting... First I am registering the device and saving the id on my server:
FirebaseInstanceId.getInstance().getInstanceId()
    .addOnCompleteListener(new OnCompleteListener<InstanceIdResult>() {
        @Override
        public void onComplete(@NonNull Task<InstanceIdResult> task) {
            if (!task.isSuccessful()) {
                Log.w(TAG, "getInstanceId failed", task.getException());
                return;
            }
            // Pega o ID token
            String token = task.getResult().getToken();
            // depois envia para o servidor
            saveToken(token);
        }
    });
So I try to send my message through the back-end to:
https://fcm.googleapis.com/fcm/send
With the id that was registered in the database
As in this example tutorial: