We automatically add an unsubscribe link snippet to your marketing emails by default. This would look something like below however, you can change the text to whatever you like:
You can also add the unsubscribe link by clicking the the Fields button in the WYSIWYG editor. Choose Common Fields as category and click the Unsubscribe Link to automatically insert the link into your campaign.
Beyond that, you can also create your own custom unsubscribe templates. You may learn more about this in our documentation here.