I manage a mailing campaign with mailjet API.
I can create a campaign and send it with $mj->post
to a contact list.
I've tried to add an existing template (which has unsubscribe link) to the newsletter but it's impossible.
So I don't know how to manage unsubscribe link inside the newsletter.
According to your message, I understand you want to have the unsubscription link inserted in the body of your newsletter.
If this is the case, then you have to mention [[UNSUB_LINK]]
in the newsletter body (see here for full instructions).
When sending/testing, Mailjet system will replace the tag with the correct value, in the locale specified in the newsletter resource.
Hope it helps.
Arnaud from Mailjet.