Best answer by dov.derin
Thank you for sharing your question with the Klaviyo Community.
The simplest solution is to embed a Klaviyo sign-up form on the Webflow page. You can embed a full subscribe page on your site pointing to the list if you wish.
Alternatively, you can add users to a list using a POST request from the third-party signup form. API requests are made to the Klaviyo Subscribe URL endpoint: https://manage.kmail-lists.com/ajax/subscriptions/subscribe. POST requests to the above endpoint add a user's email and additional user properties to any list in Klaviyo that you choose. This endpoint is unique from the standard subscribe endpoint used in Klaviyo's native signup forms. We have more information on the POST body in our documentation here.
Thanks and have a great day.