put
https://api.sintalk.com.br/v3/contact-lists//contacts/
Adds a contact you already have, by id, to this list. There is no request body: only the membership is created, the contact's own fields are left untouched, and repeating the call changes nothing. Both the list and the contact must already exist, or the call is 404.
When you hold a phone number instead of an id, or want the contact created if it does not exist yet, use POST /contact-lists/{list_id}/contacts.
204The contact is in the list; no body is returned.