First thing is adding the content below the first article.
I see you have an Advert already in that place? If you used the Ad Inserter Plugin:
https://en-gb.wordpress.org/plugins/ad-inserter/
Then this also allows you to add HTML and other types of content.
You can then use the position: sticky; property to make it stick on scroll.
Example HTML:
<div class="sticky-element">
<!-- my sticky element HTML goes in here -->
</div>
Then this CSS to make it stick:
.sticky-element {
position: sticky;
position: -webkit-sticky;
top: 0;
}