title: AMP Form submission redirect or response
body: <p>Good morning,
  I have a form on a test AMP page (is AMP validate) and the form works: I receive the email with result (I use a php to handle it).
  I don't know (I did some try but I think I still missing an example) the syntax for let the AMP page responce correctly (now I get "Subscription failed!" but I do get the email) or redirect after submission.
  Here my example:</p>
  
  <p><a href="https://pizzaoventhailand.com/TMP/contattaci_standby_AMP.htm" rel="nofollow noreferrer">AMP page with form</a> (I receive form result after submission but I don't know how to redirect or get "Subscription successful!" message)</p>
  
  <p><a href="https://pizzaoventhailand.com/TMP/contattaci_standby_NO_AMP.htm" rel="nofollow noreferrer">Non AMP page with form</a> (I receive form result and it redirect correctly)</p>
  
  <p>action-xhr file destination code: 
  <a href="https://pizzaoventhailand.com/TMP/form_AMP_EN_nocaptcha_3.php.txt" rel="nofollow noreferrer">Code of the php here in txt format</a> (the file that handle the form result)</p>
  
  <p>Any example will be great for me also only about redirect.
  Thank you</p>