How to call slack API

slack.js

// See https://developers.google.com/apps-script/guides/services/external
// and https://developers.google.com/apps-script/reference/url-fetch/url-fetch-app
function fetchListOfChannels() {
  var options = {
    'method' : 'get',
    'contentType': 'application/json',
    'headers': {
        'Authorization': 'Bearer <access_token>',
    }
  };

  var url = "https://slack.com/api/conversations.list"
  + "?types=private_channel" 
  + "&pretty=1";
  var response = UrlFetchApp.fetch(url, options);

  var json = response.getContentText();
  var data = JSON.parse(json);
  Logger.log(data);
  for (var i = 0; i < data.channels.length; i++){
    Logger.log(data.channels[i]);
  }
}