Xenforo 2.1 How to add html to thread message in specific node categories?

[dark_night]

how can i be able to add text/html in message in certain node categories? i would like to add a button to the top of only the first post in the thread in specific node categories. what would be the best way of doing this? or would this require an addon? i remember being able to do this with conditionals in mybb, but not sure about xenforo