How to pin a conversation
Go to the menu Add-on > Forum > Threads.
From there you will select the thread of which you which to pin the message and click on edit(1).
Scroll to the bottom of the threads page and you will see the list of the conversations(2).
Hover over the conversation that you would like to pin.
An options bar will appear under the conversation. You have the chance to Edit post, Pin the message, Delete this message, Spam(3).
You can choose to pin and unpin this message