Firebase Cloud Function Integration with Payment Gateway (BillPlz)

Question

so I have been trying to integrate Firebase Cloud Function (Node.js) to the Billplz payment gateway. I am able to authenticate and create the bill however I am having difficulties to read the response data (mainly the url to which I would need to send to my App to redirect them to the Payment Page).

I am using fetch function to POST to the collection to authorize and create a bill and not sure on how to extract the response data. I know the response is as a JSON object but I am having difficulties extracting the data.Hope someone here can help me with this.

auth_promise().then((auth_res)=>{
    if(auth_res.status != 200){
      console.log(fail);
      throw Exception('http.post error: statusCode='+ auth_res.status);
    }else{
      console.log('Authentication - '+success);
      bill_promise().then((bill_res)=>{
        bill_res.json();
      }).then((data)=>{console.log('data: '+ data)});
    }
  });
});