Google Apps Script: A Beginner's Guide To Learn Apps Script

// custom menu function
function onOpen() {
  var ui = SpreadsheetApp.getUi();
  ui.createMenu('Custom Menu')
      .addItem('Save Data','saveData')
      .addToUi();
}

// function to save data
function saveData() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var sheet = ss.getSheets()[0];
  var url = sheet.getRange('Sheet1!A1').getValue();
  var follower_count = sheet.getRange('Sheet1!B1').getValue();
  var date = sheet.getRange('Sheet1!C1').getValue();
  sheet.appendRow([url,follower_count,date]);
}