how to call a php function with ajax code example

Example: ajax call too functions.php

<?php
    // ajax call to this function
    function register_user()
    {
        $username = $_POST['username'];
        echo $username;

        die();
    }

    add_action('wp_ajax_register_user', 'register_user');
    add_action('wp_ajax_nopriv_register_user', 'register_user');
    
?>