Use Klaviyo as an access control list
By default, our Klaviyo key condition signs up your customers for your Klaviyo mailing list, when they submit their email address. However, this key condition also supports using Klaviyo to establish a list of email addresses who are preauthorized for your content.
Once you've created a lock, select "subscribes to your Klaviyo list" from the list of available key conditions.
Then, after choosing your Klaviyo list, check the box labeled "Only grant access if the customer is already on this list".
As noted in the form, with this mode enabled, Locksmith won't add anyone to your Klaviyo list. Instead, access will only be granted for confirmed email addresses that are already on your list.
You may also want to edit the prompt under Messages > "Mailing list signup prompt" further down on the page - to reflect the fact that you're checking for preauthorized access, instead of looking for signups!