Hi @Francois123,
Contact Form 7 does not have a Klaviyo-approved integration at this time, but you will be able to subscribe users via our API. You'll specifically want to use List API V2 in this doc: Klaviyo's REST API References.
Additionally, when using the List V2 API, users will have to confirm their subscription via our double opt-in process, before being added to the list. You can always disable the double opt-in setting in the list settings. For more information, please see our article on this here.
Alternatively, you can replace the form with one of Klaviyo's. Take a look at our form instructions in these linked guides: Create a Signup Form from the Form Library and Guide to Creating a Signup Form.
Thanks for being a member of the community!