GAS script that find specific email from Gmail, extract body from it, then send it to a slack channel

main.gs

var token = "...";
var channelId = "...";

function run() {
  
  var threads = GmailApp.search('subject:"send-to-slack" label:inbox', 0, 500);
  if (threads.length == 0) {
    return;
    
  } else if (threads.length > 1) {
    Logger.log("Illegal number of threads.");
    return;
  }
  
  var thread = threads[0];
  if (thread.getMessageCount() != 1) {
      Logger.log("A thread should have one message.");
      return;
  }
    
  sendToSlack(thread.getMessages()[0].getPlainBody());
  thread.moveToTrash();
}

function sendToSlack(message) {
  
  var params = [
    "token=" + token,
    "text=" + message,
    "channel=" + channelId,
    "as_user=true"
  ];

   var resp = UrlFetchApp.fetch("https://slack.com/api/chat.postMessage?" + params.join("&"));
}