Take the code from this form! This means you should take all the hidden and strange Marketo values as well. You can then style this form and add your own validation if you like. Don't modify the hidden fields at all. 

Make sure to also include the Marketo Tracking Script:

<script type="text/javascript">(function() {  var didInit = false;  function initMunchkin() {    if(didInit === false) {      didInit = true;      Munchkin.init('564-SZS-136');    }  }  var s = document.createElement('script');  s.type = 'text/javascript';  s.async = true;  s.src = '//munchkin.marketo.net/munchkin.js';  s.onreadystatechange = function() {    if (this.readyState == 'complete' || this.readyState == 'loaded') {      initMunchkin();    }  };  s.onload = initMunchkin;  document.getElementsByTagName('head')[0].appendChild(s);})();</script>