I want to automatically assign the username of the executor in GAS

function onOpen2(){
  varurl='https://chat.googleapis.com';
  var text = 'Automatically insert viewer's name';
  var payload = {
    "text"—text
    }
  varjson=JSON.stringify(payload); 
  var options = {
    "method": "POST",
    "contentType": 'application/json; charset=utf-8',
    "payload": json
  }

  var response=UrlFetchApp.fetch(url, options);
  Logger.log(response);

}