Add the script in your site's custom code, then put the box in a Code Embed.
Site settings → Custom code → Footer code
<script src="https://askthissite.com/embed.js" async></script>
A Code Embed element where the box should show
<div data-everypage-site="YOUR-SITE-ID"></div>
Custom code needs a paid Webflow site plan.
Your site id is on your site's page after you sign up.