Hi @smp,
Thanks for sharing this question with the Klaviyo community.
I’m afraid you cannot use an identify call to subscribe a user to a specific list. Instead, you should be using the subscribe endpoint: https://a.klaviyo.com/api/v2/list/{list_id}/subscribe. After this request is made, by default, the user being subscribed will receive a double opt-in email asking them to confirm their subscription. Once confirmed, they will be added to the list specified in the call. We have an example request in our API documentation on that here.
Also keep in mind, there are plenty of other ways to subscribe users to lists in Klaviyo. Including adding them via a .csv file. This is ideal if you have a lot of users to add to a list at once. Or, you can quick add users to a list, an ideal method if you only have a handful of users you’d like to add to a list. And of course, using Klaviyo sign-up forms or a subscribe page.
I hope that’s helpful.