You will need to add a placeholder link or text to your email. The format of that placeholder depends on your Email Service Provider.
For example, in MailChimp the placeholders are called Merge Tags.
You can read more about it here.
MailChimp is using the tag *|UNSUB|* for unsubscribe links.
If you want to add a dynamic unsubscribe link to your email under a text:
You need to select the desired word(s)
Click the link toolbox
Choose Protocol type <other>
Type the desired merge tag into the URL field
Let us know if you have any other questions!