<a href="<?php echo add_query_arg( array( 'page' => 'wpbs-calendars', 'subpage' => 'upgrade-to-premium' ), $this->admin_url ); ?>" class="wpbs-wrap-upgrade-cta">
    <span class="wpbs-wrap-upgrade-cta-button">See details</span>
    <span class="wpbs-wrap-upgrade-cta-heading">User notifications are only available in the premium version.</span>
</a>