You can always use the following BB Code in order to display a YouTube clips within your post instead of having an external link:
HTML Code:
format: [youtube]clip's ID[/youtube]
example: [youtube]zZe9OtFNt_8[/youtube]
There is also a special BB Code for YouTube HD:
HTML Code:
format: [youtubehd]clip's ID[/youtubehd]
example: [youtubehd]OHehm0HDLHU[/youtubehd]
For your convenience, there are two appropriate buttons in the toolbar of Message Editing part:
