The "flag" link, if enabled (not available for accounts with <500 karma) appears when you open a comment directly.
Otherwise, for this or any other request, email mods at hn@ycombinator.com. "@dang" is a no-op, and replies-to-comments are often overlooked: <https://news.ycombinator.com/item?id=39171237>.
FWIW, 500 karma is the threshold for downvoting, but I'm pretty sure the threshold for flagging is significantly lower, unless something has changed in the years since I was a new account.
Otherwise, for this or any other request, email mods at hn@ycombinator.com. "@dang" is a no-op, and replies-to-comments are often overlooked: <https://news.ycombinator.com/item?id=39171237>.