"Get Started" button not working in Facebook Page

Question

I recently setup the "Get Started" button using the following POST method using Postman:

https://graph.facebook.com/v2.6/me/messenger_profile?access_token=<my token>

With the raw text:

{
  "get_started": {"payload": "<postback_payload>"}
}

It returns:

{
    "result": "success"
}

But nothing show up in my Messenger (deleted the chat already)

If I send the raw data to setup the greeting text, it works normally and showed up:

{
  "greeting": [
    {
      "locale":"default",
      "text":"Hello!" 
    }, {
      "locale":"en_US",
      "text":"Timeless apparel for the masses."
    }
  ]
}

Can somebody help me, thanks!