How to send multiple input field data via post in ajax and javascript?

Question

I can submit one input field data through ajax to a php file and then that php file update it in database. Last portion of code i write for ajax in javacript is below

parameter = 'create_name='+document.create_user_cl.create_name.value;
    xmlhttp.open('POST', 'php/add_user.php', true);
    xmlhttp.setRequestHeader('Content-type', 'application/x-www-form-urlencoded');
    xmlhttp.send(parameter);

Everything works uptil here.

Now I have multiple input fields like name, username, password etc and i want to submit all input field's data via post through Ajax to the php file but i don't know how should i modify that parametervariable or xmlhttp.send() so that i can send multiple input field data. I can do it with GET but i want to achieve this through POST. I am new to ajax, please help