There are two Liquid tags that will assist you in generating unique unsubscribe links for your messages. {{ unsubscribe_link }} and {{ unsubscribe_url }}. They work differently and which one you use will depend on what you need in your message.
{{ unsubscribe_link }}
This tag generates the entire link for you and should be placed anywhere you want us to insert our default unsubscribe link. The default unsubscribe link will always say "unsubscribe" and it will always be untracked automatically. When using the {{ unsubscribe_link }} tag you never need to add anything to the tag to ensure that it is untracked.
{{ unsubscribe_url }}
This tag generates just the URL to be used in a link (rather than generating the entire code for the link) and should be placed anywhere you would normally place the URL for the link you want to use for your link.
When using the code and rich text editors, that means placing the {{ unsubscribe_url }} tag in the href attribute of an <a> element in the HTML as shown below:
<a href="{{ unsubscribe_url }}">Click this custom text to unsubscribe!</a>