Klaviyo and other email marketing platforms use customer tags in order to sync newsletter subscribers to lists that can be used for email marketing campaigns. You may find the following instructions that indicate the requirement of a newsletter tag in order to sync your subscribers to a Klaviyo list.
The good news is this has already been done for you in our themes. No additional changes are needed. Any visitors who subscribe to your newsletter will automatically receive the newsletter tag that you'll see when viewing your customers in the Shopify admin. Klaviyo will automatically sync these subscribers based on that tag being applied.
For any further integration questions or support, please refer to Klaviyo's documentation.