Note that this guide is for videos uploaded to YouTube.
When posting on the forums, you should see a window similar to the following:
click the "youtube" button as shown.
Between the two markers added to your post, put the video ID of the YouTube video you would like to share [Note: this is not the full URL, but the ID part of the url]
In this case, the video ID is the circled part of the URL: "o6DHQWOEfqM"
Once you're done, it should look something like this:
Be sure to make use of the "PREVIEW" button to see what your post will look like before finalizing it with the "SUBMIT" button