// Start your PHP snippet with a ?>
?>
<!-- Content. Edit as you wish. -->
<h3>Popup Title</h3>
<p>Lorem ipsum dolor bla blah.</p>

<!-- Edit only those necessary. -->
<form method="post" action="<?php echo esc_url( home_url() ); ?>">
    <input name="accept" type="checkbox" value="Gdpr" required /> I accept the GDPR
    <input type="submit" id="accept-button" value="Accept" />
    <?php wp_nonce_field( 'accept-gdpr', 'accept_gdpr_nonce' ); ?>
</form>