How do I create a FeedItem post with an @collaborationgroup

I've created a feed item post without issue but setting the @ in the body text does not work. How do I accomplish this

Happening on a case object

{
  FeedItem fi = new FeedItem();
  fi.parentid = c.id;
  fi.type = 'AdvancedTextPost'; // I've tried a few variations here
  fi.visibility = 'AllUsers';
  fi.body = '@Level-1-Updates Level 1 Case DETECTED for: ' + c.account.name;
  lfi.add(fi);          
}
insert lfi;

The problem is the feed item is plain text the collaboration group @Level-1-Updates is not getting notified or treated as a collaboration group. It's treating it as plain text