Sadly that code snippet doesn't work. Here's what happens,
The URL to the thread I'm on is = https://thefanfictionforum.net/xenforo/index.php?threads/other-options-for-posting.21589/
After clicking the button (using suggestion) I come to,
https://thefanfictionforum.net/xenforo/reply
Which...
Members on my forum would find it useful to write their replies on a separate page, instead of right below the threads. A bit of google-ing and I found this,
https://xenforo.com/community/threads/xf2-full-page-text-editor-for-replies.140653/
According to that thread, what I need to do, is...