How to detect a hangouts card click event in DialogFlow using webhook?

Question

I have created a Dynamic card in Dialogflow that should display in hangouts. Here is the custom card i have created in Dialogflow responses under custom payload:

{
  "hangouts": {
    "header": {
      "imageUrl": "imageurl",
      "title": "Hi, Select your choice ?"
    },
    "sections": [
      {
        "widgets": [
          {
            "buttons": [
              {
                "textButton": {
                  "text": "Card",
                  "onClick": {
                    "action": {
                      "actionMethodName": "CardResponder"
                    }
                  }
                }
              },
              {
                "textButton": {
                  "text": "Cash",
                  "onClick": {
                    "action": {
                      "actionMethodName": "CashResponder"
                    }
                  }                  
                }
              }
            ]
          }
        ]
      }
    ]
  }
}

The card looks like this in hangouts. card

In the webhook code my entry point is this:

app.post('/', express.json(), (req,res)=>{
    if(req.body.queryResult.intent.displayName == "PaymentIntent"){
        ........
    }    
});

I am not able to detect the event when i click on Cash or Card button on my hangouts dynamic card. I have defined function name as well in the actionMethodName parameter. But I am not sure how to write that function in my webhook code. Please help me.

Update

app.post('/', express.json(), (req,res)=>{
    console.log(req.body);
    console.log(req.body.action.actionMethodName);
    
});

I am trying to console req.body which gives me

originalDetectIntentRequest: { source: 'hangouts', payload: { data: [Object] } },

req.body.action.actionMethodName gives undefined.

When i try to click button in hangouts card it gives me this error - Unable to contact hangouts-bot. Try again later.

hangouts-bot is the name of my cloud project. I do not know how to recieve the click request in my node js code.