Passing on metadta via REST

Hello I have a small problem with passing on metadata.

The REST request to /webhooks/rest/webhook is:

 {
  "sender": "12345",  
  "message": "how is the world",
  "metadata": {"category": "test"}  
}

I try to extract the metadata in the actions with: tracker.latest_message.get(“metadata”) This always returns []

If I use only look at: tracker.latest_message The return contains:

{‘intent’: {‘id’: 1172471429406228305, ‘name’: ‘world’, ‘confidence’: 0.9986758828163147}, ‘entities’: [], ‘text’: ‘how is the world’, ‘message_id’: ‘89b88d704e1e4d21a42128b5c9c947fa’, ‘metadata’: {}, ‘intent_ranking’:

So the metadata JSON is empty.

Is there a mistake in the REST-request, do I need to create a custom connector or is there another error?