You are new to HN. If you click on the timestamp of a comment, you go to the permalink for that specific comment. There, there is a "favorite" button you can us

You are new to HN. If you click on the timestamp of a comment, you go to the permalink for that specific comment. There, there is a "favorite" button you can use to save it. You can access all the comments you have upvoted or favorited from your profile page.