BroadcastSubscriberGroupMessage

{
    "clientMessageId": "3cb69db1-c410-4b83-8d37-84b823be9c06",
    "message": {
        "allowSelect": true,
        "text": "Hello World",
        "attachments": [
            {
                "type": "image",
                "imageUrl": "https://images.pexels.com/photo.jpeg"
            }
        ]
    }
}