CBSubs tab does not allow fields. It's not possible to add fields to it without modifying the tab row in database and setting its fields column to 1, but that's just a temporary fix as running CB Tools or updating CBSubs will set it back to 0. I recommend just adding your text to the description of the plans. You can use substitutions in the descriptions so you can condition to see if the user is logged in or not if you want it on registration only.