Javascript Bitly API Post请求

我试图使用下面的代码向Bitly API发出请求，但收到422错误消息。我没有使用OAuth，因为像我这样的单帐户用户似乎不需要它

文档中没有太清楚地说明在这种情况下如何发出请求

function bitly() {
  var long_url = "https://stackoverflow.com/questions/ask";
  var apiv4 = 'https://api-ssl.bitly.com/v4/shorten';
  var genericAccessToken = "xxxxx";

  var params = {
  method: "post",
  headers: {"Authorization" : "Bearer " + genericAccessToken, 
            "Content-Type": "application/json"},
  payload: {"group_guid": "string",
            "domain": "bit.ly",
            "long_url": long_url},
  };
  var res = UrlFetchApp.fetch(apiv4, params);
  var obj = JSON.parse(res.getContentText());

}