How to send Postback Payload in Webhooks?

Summary

Is it possible to update the variables using a postback payload from a webhook channel?

Content

Hi developers,
I am developing a custom SDK for Hangouts Chat and I want to update the variables using Postback actions.
I referred to the official documentatiton and I am a little unclear of what needs to be done. 
Could anyone give an example of a message payload for Postback action that updates the conversation variables?
A full example is really appreciated. 
This is my code snippet:

 

let message = {

"userId": userEmail,

"profile": {

"firstName": "Bob",

"lastName": "Franklin",

"age": 45

},

"messagePayload": {

"type": "postback",

"postback": {

"variables": {

"name"