How to custom facebook webview card, which allows "default_action", "messenger_extensions" and "webview_height_ratio" in dialogflow fulfillment?

Question

I am customizing the facebook card in my fulfillment. I expect my card can open a webview as a popup. As facebook says, I have to set "messenger_extensions" to true (see below)

"payload": {
  "template_type":"generic",
  "elements":[
     {
      "title":"<TITLE_TEXT>",
      "image_url":"<IMAGE_URL_TO_DISPLAY>",
      "subtitle":"<SUBTITLE_TEXT>",
      "default_action": {
        "type": "web_url",
        "url": "<DEFAULT_URL_TO_OPEN>",
        "messenger_extensions": <TRUE | FALSE>,
        "webview_height_ratio": "<COMPACT | TALL | FULL>"
      },
      "buttons":[<BUTTON_OBJECT>, ...]      
    },
    ...
  ]
}

However, when I add this template to my card, I get no response from dialogflow.

{
    fulfillmentMessages: elements, // elements works and returns cards
    payload: { 
      "facebook": { // THIS PART DOES NOT WORK
          "attachment": {
            "type": "template",
            "payload": {
              "template_type": "generic",
              "elements": [{
                "title": "Welcome!",
                "image_url": productCover,
                "subtitle": "We have the right hat for everyone.",
                "default_action": {
                  "type": "web_url",
                  "url": webviewURI + psId,
                  "webview_height_ratio": "tall",
                  "messenger_extensions": true
                },
...

Thanks