In the case of your post just now, you want to click the "2018-04-14 22:23:01" part. If you hover over it, it should say "Permalink to this item". After you click it, the thread title at the top of the page should say "Parent: What are you hyped about?" rather than just "What are you hyped about?". Once you're there, you can reply normally in the "Your reply" window.