The Classic Builder doesn't include a fixed side banner element, but you can pin a box element to the side of the browser window with custom CSS so it stays in place as the visitor scrolls.
This is community code, not an official Unbounce feature. Unbounce Support can't troubleshoot custom code. Before you add a side banner, consider whether it suits your page. On many landing pages, a single focused call to action converts better than extra sidebar content.
Pin a box element to the side
Add a box element to your page, then add the following to the Stylesheets section. Replace the ID with your box's ID:
<style>
#lp-pom-box-123 {
position: fixed;
top: 0;
left: 20px;
width: 320px;
height: 100%;
}
</style>
To find the box ID, publish the page and inspect the element in your browser's developer tools. It follows the pattern lp-pom-box- followed by a number. Adjust left or right, width, and top to position the banner.