If you do not have participation rights in a discussion (see: Participant Rights and Roles), you will not be able to comment on a claim directly. Instead, if you follow the same steps to Commenting on a Claim, you will be able to suggest a claim which can be accepted by any discussion Admin. Until a suggested claim is accepted, it is only visible to you and discussion Admins.
Once any Admin in the discussion accepts your suggested comment, it will be visible to all users in a discussion as a regular comment would. Admins can also respond to your suggested comment, for example, if they feel it might need clarification, and you can edit and resubmit your comment if it is not initially accepted.