Hello,
Sorry, but the Klaviyo service is related to the third-party plugins and without additional plugins to the theme, it won’t work, also we don’t have such information on how to use it.
I suggest you set any “Newsletter” template from the “XTemos templates” in the HTML block and define it via Theme Settings -> General -> Promo popup.
https://monosnap.com/file/rzplcdvtwUxLMwhmiY7CswwEtuUeWK
https://gyazo.com/13e0b39ccbb315c7b03926cc4d4b3fb4
Please let me know if you have any further questions in the meantime.
Kind Regards