I think the only answer is: You earn the requisite reputation, then post the comment.
Users who flout this convention (by posting a comment as an answer or editing existing posts) will usually have their posts flagged or edits declined. Flags require a site moderator to take the time to process the flag -- it is not a good use of time.
Requests to lower the comment-rep-requirement have been declined by the StackExchange team. See this meta.SO question as an example: Lower the amount of reputation needed to comment
Here is a full list of reputation privileged -- 50 rep is not a terribly difficult milestone to reach. Even just asking questions earns you rep.