GetGmailEmails.gs

var ui = SpreadsheetApp.getUi();
function onOpen(e){
  ui.createMenu("Gmail Manager").addItem("Get Emails", "getGmailEmails").addToUi();
}

function getGmailEmails(){
  var label = GmailApp.getUserLabelByName('test');
  var threads = label.getThreads();
  for(var i = threads.length - 1; i >=0; i--){
    var messages = threads[i].getMessages();
    for (var j = 0; j <messages.length; j++){
      var message = messages[j];
      extractDetails(message);
    }
    threads[i].removeLabel(label);
  }
}

function extractDetails(message){
  var dateTime = message.getDate();
  var subjectText = message.getSubject();
  var senderDetails = message.getFrom();
  var activeSheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();
  activeSheet.appendRow([dateTime, senderDetails, subjectText]);
}

1. How can I get emails automatically instead of the manual button clicking whenever new emails arrive?
2. Also help to get a unique email with a unique subject line that arrived first in the labeled.