As is required to comply with GDPR law, your promotional newsletters must always have an unsubscribe link, to allow the contact to easily remove themselves from the system.
Our pre-built templates all have this by default, but if you create your own, simply create a link in the drag & drop editor with {{unsubscribe}} as the target.
Clicking on the link icon opens the link pop-up window:
When the e-mail is sent, this automatically will display the real link to allow a contact to unsubscribe successfully.