Here's a tutorial for linking / embedding videos in a topic thread. For the most part this is based on working from YouTube, but the same idea applies to any video streaming site.
Start by typing your text into the editor, but make sure you turn on the "Advanced Editor" by clicking the underlined "A" in the upper right corner:
Then you want to click on the film strip icon:
Another way is to right click, then Copy the URL highlighted in blue;
Go back to the message area and click the Insert Video, paste the Youtube URL and click OK followed by the Save button. Post the reply as you normally do.