How to get a thread_ts as a response from the Slack API using Google Appscript? Here's my code.

function postToSlack() {
  var payload = {
    channel: "#slackbot-test",
    username: "my test",
    icons_url: "",
    text: "<@username>testing<#>",
  };
  var options = {
    method: "post",
    contentType: "application/json",
    payload: JSON.stringify(payload),
  };
  response = slack_client.chat_postMessage(
    (channel = receiver),
    (text = message)
  );

  console.log(response.data);
  thread_ts = response.data["ts"];
  console.log(thread_ts);
  console.log("Message sent: ", payload);

  return UrlFetchApp.fetch(webhookUrl, options);
}

Slack API Google Appscript