Steps to Reproduce
Go to Settings and select the type of integration (Klaviyo or Mailchimp).
Enter your API key for Mailchimp or Klaviyo.
Below the input field, you will find a link with instructions on how to generate and use your key.
Navigate to the desired Form → Form Integration → Mail Integration.
Toggle Enable/Disable Mail Integration.
Select the integration you want to use:
Mailchimp: choose the Mailchimp list where you want to add subscribers, then click Save changes.
Klaviyo: choose the Klaviyo list where you want to add subscribers, then click Save changes.
Note: Please select the "Single opt-in" process in the Klaviyo list settings to subscribe users based on the "accepts_marketing" checkbox in your form.