title: how to setup "Get started" button in facebook messenger bot and when to send welcome message
body: <p><a href="http://i.stack.imgur.com/yWt58.png" rel="noreferrer">"Get Started"</a></p>
  
  <p>tried sending request to this url </p>
  
  <p><a href="https://graph.facebook.com/v2.6/PAGE_ID/thread_settings?access_token=PAGE_ACCESS_TOKEN" rel="noreferrer">https://graph.facebook.com/v2.6/PAGE_ID/thread_settings?access_token=PAGE_ACCESS_TOKEN</a>  </p>
  
  <p>But didn't work.</p>