API v2 - Invalid grant_type requesting token

I am following the steps from OAuth 2.0 box guide (https://box-content.readme.io/docs/oauth-20).

First I authorize my application using this URL:

https://account.box.com/api/oauth2/authorize?response_type=code&client_id=&redirect_uri=http://localhost:8081/callback&state=security_token 

Then, I request the token to https://api.box.com/oauth2/token using this body.

{
  "grant_type": "authorization_code",
  "code": "",
  "client_id": "",
  "client_secret": ""
}

The code is the same as the first step return me at the URL.

But I always get the same error:

{
    "error": "invalid_request",
    "error_description": "Invalid grant_type parameter or parameter missing"
}

What can solve that problem?