There are two ways to add the Subscribe tag to your campaigns:
1. You can place the tag *|LIST:SUBSCRIBE|* in the body of your campaign and the tag will get replaced with your subscription link for the list to which you sent that particular campaign.
Before
After
2. You can also place the tag into your code to hide the link. To do so you'll need to place this code directly in the SOURCE area of your campaign.
<a href="*|LIST:SUBSCRIBE|*">Subscribe to our mailing list.</a>