showcase/_includes/sidebar.html

<div class="sidebar sticky-top">        
    <div class="sidebar-section">
        <h5><span>JOIN US</span></h5>
        {% include newsletter.html %}
    </div>
    <div class="sidebar-section">
        <h5><span>KNOW MORE</span></h5>
        <a href="https://codeninjaslangley.co.uk" target="_blank"><img src="{{site.baseurl}}/assets/images/cn-advert.png"></a>
    </div>
</div>