How to use a frontend URL with a Plugin

Must exist a real page in WordPres?

Yes, you need to create a page, but it doesn’t have to have any content.

Let’s say you created a new page named ‘Confirmation message’. The URL would be http://example.com/confirmation-message.

Now add the following to the functions.php of your theme:

/**
 * Adds a confirmation message to the content of a page.
 * 
 * @param   string  $content    The current content of the page.
 * @return  string              The new content of the page.
 */
function show_confirmation_message($content) {

    // Don't add message if 'myplugin', 'confirmkey' and 'mail' are not in the URL.
    if (!isset($_GET['myplugin']) || empty($_GET['confirmkey']) || empty($_GET['mail'])) {
        return $content;
    }

    // Don't add message if we're not on a page and not in the main query.
    if( !is_page() || !is_main_query() ) {
        return $content;
    }

    // Add the logic to show a confirmaiton message here... 
    $content.= 'This is the confirmation message.';

    return $content;
}

add_filter('the_content', 'show_confirmation_message');

Now, if you go to:

http://example.com/confirmation-message?myplugin&confirmkey=KEY&mail=MAIL you should see the confirmation message.