1. Edit an existing challenge/course that you have created.
2. Click on Show Advanced Options inside the details tab.
3. Scroll down to where you see Allow user tag based access to this challenge and check the box.
4. Search for the tag you want and insert it.
Now if any user has this tag they will automatically have this challenge inside their dashboard as well.
Note: Because it is provided by tag only, challenge rules (such as emails, tags, etc...) will not be added as those are only applied when the challenge is directly added to the users account.