function doGet() {

  return HtmlService.createHtmlOutputFromFile('index.html');

}

function doPost(e) {

  const message = JSON.parse(e.postData.contents);

  const sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();

  sheet.appendRow([message.sender, message.text, new Date()]);

  return ContentService.createTextOutput("Message received").setMimeType(ContentService.MimeType.TEXT);

}