• Select the text you want to use as hyperlink.
  
  
• Click on the "Add Hyperlink" icon.
  
  
• Now change the type to "mailto:".
  
  
• Enter the URL. This should be mailto: followed by your list email and the subject keyword to unsubscribe.
  
  

Sample:

Lets say the email address of your list is yourlist@yourdomain.tld and the unsubscribe keyword is the default (Remove).
mailto:yourlist@yourdomain.tld?subject=remove

A click on mailto: link will open the recipients mailclient with pre-set recipient and subject, so the member only has to send it.
The message with the subject "remove" would be automatically received by the program through the next "Synchronize".

Alternative:

• Use a link to a webform which generates the unsubscribe email
  
  
• Use "Reply with Remove in the subject" instead of the mailto link
  
  
• Use a combination e.g. Click here to unsubscribe (as mailto link) ... or reply with "Remove" in the subject