Add Subscriber to Form
Add an existing subscriber to a form by their IDs. This action associates a subscriber with a form in Kit. The subscriber must already exist in your Kit account (use KIT_LIST_SUBSCRIBERS to find them). The form must also exist (use KIT_LIST_FORMS to find valid form IDs). Returns HTTP 201 if the subscriber is newly added to the form, or HTTP 200 if they were already associated with the form. Both cases are considered successful operations. Important: This action requires both form_id and subscriber_id (aliased as 'id' in requests). An optional referrer URL can be provided for tracking purposes and will have its UTM parameters automatically parsed.