using twitter api to get token

I am trying to use the twitter api, but need to get authentication. There are 2 types , and I only need Application-only authentication aka app only. This is the type of authentication where an application makes API requests on its own behalf.

The docs explain to use this method, you need to use a bearer token. You can generate a bearer token by passing your consumer key and secret through the POST oauth2 / token endpoint.

Here is the link to docs explaining this endpoint. There is even an example request but still it isn't very clear to me what needs to be done.

I have an API key and API secret key, but am getting the following error:

body: ‘{“errors”:[{“code”:170,“message”:“Missing required parameter: grant_type”,“label”:“forbidden_missing_parameter”}]}’ }

My server side code looks like this

var request = require('request');
var btoa = require('btoa');

const KEY = encodeURIComponent('1234');
const SECRET = encodeURIComponent('5678');

request({
    headers: {
      'Authorization': 'Basic ' + btoa(`${KEY}:${SECRET}`),
      'Content-Type': 'application/x-www-form-urlencoded;charset=UTF-8'
    },
    uri: 'https://api.twitter.com/oauth2/token',
    method: 'POST',
    body:  JSON.stringify({
      'grant_type': 'client_credentials' // I am passing the grant_type here
    })
  }, function (err, res, body) {
    console.log('res', res)
  });

The CURL request in the docs looks like the following:

POST /oauth2/token HTTP/1.1
Host: api.twitter.com
User-Agent: My Twitter App v1.0.23
Authorization: Basic eHZ6MWV2R ... o4OERSZHlPZw==
Content-Type: application/x-www-form-urlencoded;charset=UTF-8
Content-Length: 29
Accept-Encoding: gzip

grant_type=client_credentials