Last modified: 2014-01-08 05:28:32 UTC
Unlike other special pages, Special:UserRights does not accept GET params, so it's not possible to create user comfortable links with pre-filled items.
The username and reason can both already be pre-filled. Special:UserRights/username?user-reason=Some+reason+here. I don't think it's desirable to have the check boxes be pre-fillable, since they're the indication of the user's existing groups. I'm not sure if anything should be done here.
Closing as WORKSFORME. Reopen if there's still an issue.