To add unsubscribe link to your campaign, head over to Personalize menu of campaign body.
Use {{UNSUBSCRIBE_LINK}} to add a link with "Unsubscribe" wording.
If you want to use your own wording use {{UNSUBSCRIBE_URL}} like for example "Click here if you don't want to hear from me again."
You can do this by typing "Click here if you don't want to hear from me again." and inserting a link.
In the link editor, you enter {{UNSUBSCRIBE_URL}} into the URL field.
The final result is
Bonus tip: Use spintax in unsubscribe copy to make it as random as possible.