How to insert html/css/javascript code to wordpress plugin

I'm trying to show a modal dialog when user get registered (action_hook: user_register) using the woocommerce plugin, but without having any result.

I would appreciate any help!

The code is shown below.

myplugin.php

// handler for new_user_registered
$myplugin->register_action_hook (
    'user_register', 
    array(
        'hooksClass',
        'add_new_user'
    ));

hooks.php

public static function add_new_user ($userid)
{
    ...
    function enqueue_scripts() {
        wp_enqueue_script('modaljs', plugin_dir_url(__FILE__) . 'js/modal.js', array('jquery'));
        wp_enqueue_style('modalcss', plugin_dir_url(__FILE__) . 'css/modal.css');
    }
    add_action('wp_enqueue_scripts', 'enqueue_scripts');
    ?>
    <button id="myBtn">Open Modal</button>
    <div id="myModal" class="modal">
        <div class="modal-content">
            <div class="modal-header">
            <span class="close">&times;</span>
            <h2>Header</h2>
            </div>
            <div class="modal-body">
              <p>Content</p>
            </div>
            <div class="modal-footer">
            <h3>Footer</h3>
            </div>
            </div>
        </div>
    <?php
}

What I am getting as a result is this image

Thank you in advance for any suggestion!