First of all go to YouTube and find the video you want to post on the forum.
For example:
Then look at the URL/Address bar and select the text after watch?v=
For example:
Now on the forum go to post a reply or new thread and click the YouTube button
You can see it at the right hand side of this image:
Once clicked it will show you the tags:
Insert the text previously highlighted text in the URL of the YouTube video link e.g. jm4q9NCBVkY
Now submit your post/thread and it will appear like below: