Add the script under Appearance, then put the box on any page.
Appearance → CSS, JS, HTML → End of Body Tag
<script src="https://askthissite.com/embed.js" async></script>
Any page (HTML)
<div data-everypage-site="YOUR-SITE-ID"></div>
Custom scripts need a ReadMe plan that allows custom JavaScript.
Your site id is on your site's page after you sign up.