The idea was to send and receive it on your own (via your taxi app). So you send a FCM message from your app to your GF using the same app. You won't need Facebook.
If you really need to post a message on Facebook/Messenger, search for WhatsApp/Facebook & intents (communication between apps work via intents. It's like you share a photo on FB. The Gallery app sends an intent to FB and receives the pc. You can do it with the messenger, too. Search the www for the intent and follow the examples here).