matrix-spec/event-schemas/examples/m.room.message#m.notice

{
  "$ref": "core/room_event.json",
  "type": "m.room.message",
  "content": {
    "body": "This is an example notice",
    "msgtype": "m.notice",
    "format": "org.matrix.custom.html",
    "formatted_body": "This is an <strong>example</strong> notice"
  }
}