How to send a simple ajax call from front end to a php backend (php file) to add a value to the $_SESSION

add_to_session.php

<?php
if(!isset($_SESSION)) { session_start(); }
$username = $_POST['user_name'];
$_SESSION['usernames'] = $username;
?>

handleAjax.js

function sendAJAX(name){
  $.ajax({
			url: './add_to_session.php',
			type: 'post',
			dataType: 'json',
			data: { "user_name": "name" },
			success: function (response) {
        console.log('Username '+name+'has been added to the session' )
			}
		});
}

ui.php

<?php
echo '
$name = 'DilumDeSilva';
<button class="sample-class" id="sample-btn" onclick("sendAJAX($name)")>Click Me</button>
   ';
?>