UBB.threads is using Jack Moore's Colorbox plugin for jQuery for that:
Code
<link rel="stylesheet" property="stylesheet" href="/ubb/ubb_js/assets/colorbox.css">
<script src="/ubb/ubb_js/assets/jquery.colorbox.min.js"></script>
<script>
$(document).ready(function() {
	$(".cbpoll").colorbox({ iframe:true, maxWidth:"550px", innerWidth:"80%", innerHeight:"100px", transition:"none", fastiframe:false, overlayClose:false, close:"X" });
});
</script>
<i class="fas fa-chart-bar fa-fw fa-lg" aria-hidden="true"></i>&nbsp; <a class="cbpoll" href="/ubb/ubbthreads.php?ubb=pollmanager&amp;id=f155deb5437db1b43e92b7c0be3556f9">Poll Manager</a>
You find a documentation here: http://www.jacklmoore.com/colorbox/