No clear way to publish an action in ws-message format

[kwellman]

The documentation (https://pushpin.org/docs/usage/#subscribing) gives this example to publish a close action:

POST /publish HTTP/1.1
Host: localhost:5561
Content-Type: application/json

{
  "items": [
    {
      "channel": "mychannel",
      "id": "an-item-id",
      "formats": {
        "ws-message": {
          "action": "close",
          "code": 1013,
          "reason": "bye for now",
        }
      }
    }
  ]
}

I can't figure out a way to do this using this library besides writing my own custom message format class since the WebSocketMessageFormat class only has content parameter.