For Google Sites websites you will need to whitelist two different domains:
Whitelist the Google Sites domain https://atari-embeds.googleusercontent.com/
Whitelist your website URL (e.g. https://my-amazing-theatre.com/)
Add a new section on the page where you would like your ticket widget to appear by selecting Embed from the Insert menu.
This will show a dialog box called “Embed from the web”. Select the “Embed code” option and paste the code snippet from the OTS Dashboard.
Select Next and you should see the widget appear in your page.