Find any topic with more than one post, use either the button or the keyboard shortcut to reply to the first post, and the reply will behave as if the “Reply to Topic” button has been used. There will be no link with the first post.
This is by design. The first post is the topic definition. Quote content from the first post if more specificity is needed.
Unfortunately, because it works for other posts, people tend to expect it to work for the first post too and don’t notice that it hasn’t until confusion ensues.
It seems reasonable to expect “reply to post” to function in the same manner for all posts; if it is not intended to function for the first post, then the “reply” button there should be removed/disabled.
There is a lot of previous discussion on this. I don’t see value in duplicating that discussion here so I am closing this.
Less noisy notifications for prolific topic authors