While Klaviyo doesn't have a built-in integration with Leadpages, it's possible to sync a Klaviyo signup page through an HTML widget in Leadpages.
Connect a Klaviyo Sign Up Form to Leadpages
Follow the instructions here for adding an HTML Widget using the Drag and Drop Builder in Leadpages (available with the new user interface in Leadpages only). In Leadpages you will see the following for entering HTML code.
Insert the following code into the HTML Widget, replacing LIST_ID_HERE with the ID of the list you want to associate with the form:
<form id="email_signup" action="//manage.kmail-lists.com/subscriptions/subscribe" method="POST" class="form-newsletter" >
<input type="hidden" name="g" value="LIST_ID_HERE">
<input value="" name="EMAIL" placeholder="Enter your email Address" required="">
<input value="submit" name="subscribe" id="submitbtn" type="submit">
To find your List ID in Klaviyo, go to the Lists and Segments tab and click into the list you would like to connect to Leadpages. The List ID will be in the url:
Redirect a Leadpages Sign Up Form Landing Page
To redirect to a desired landing page when embedding a Klaviyo sign up form on Leadpages, navigate to the Lists and Segments tab, click into the list you want to add subscribers to, and click "Subscribe Pages". On the following page, click View All Pages then Confirm Your Email under the Subscribe tab.
From here, click the Redirect box on the right hand side and insert the url where you'd like a user to be redirected to after submitting the form.