Click on the timestamp of any HN comment, then a 'favorite' link will appear.

To find it again, head to your profile -> Favorite comments

If you forget to 'favorite' a comment and vaguely remember it, sometimes a google search with site:news.ycombinator.com will do the trick.

Another way is you can use the HN searchbar (at the bottom of the page), and when it loads results, switch from searching stories to searching comments. E.g. https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...