How to send reply message to Slack with GAS? - google-apps-script

Question

First of all,I'm not native English speaker.So,I'm sorry if there are some mistake in English.
I want to make the Chat-Bot which work-flow is below.
send message from Slack
add the message to spreadsheet with GAS
send 'SAVED!' from GAS to Slack
to make the flow, I wrote the code below on GAS.
function doPost(e) {
const webhookUrl = "Incoming-Webhook-URL";
const payload = {
"text":"SAVED!"
}
UrlFetchApp.fetch(webhookUrl,{
'method':'post',
'payload':JSON.stringify(payload)
})
　
var ss = SpreadsheetApp.openById('Spread-Sheet-ID');
const sheet = ss.getSheetByName("シート1")
var token = 'Outgoing-Webhook-Token' //outgoing token
try{
// Googleスプレッドシートにデータを追加する処理
if (token == e.parameter.token){
let datetime = new Date();
let date = (datetime.getFullYear() + '/' + ('0' + (datetime.getMonth() + 1)).slice(-2) + '/' + ('0' + datetime.getDate()).slice(-2))
let time = (('0' + datetime.getHours()).slice(-2) + ':' + ('0' + datetime.getMinutes()).slice(-2));
let text = e.parameter.text;
array = [date,time,text];
sheet.appendRow(array);
}
}catch(err){
Logger.log(err)
}
}
But,The Code above doesn't send the message "SAVED!" to Slack.
On the other hand,The message is send when I push the execution button on GAS.
I can't understand why this differ is occured. Can anyone explain the reason about this please? Thank's.