Send POST parameters AJAX without jQuery

Question

I would like to send POST parameters to a controller through AJAX but without jQuery. How should I do?

function myFunction() {
    var xhr = new XMLHttpRequest();
    xhr.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
            document.getElementById("myDiv").innerHTML = this.responseText;
        }
    };
    //Add the POST parameters
    xhr.open("POST", "my_controller.php", true);
    xhr.send();
}

<button type="button" onclick="myFunction">Send POST parameters</button>
<div id="myDiv"></div>

Thank you for your help.