Right click it while it is playing and select copy video url.
Place the cursor in your post where you want the video to be and click the youtube button above your post text box.
You'll see: [youtube][/youtube]
Insert the reference number (only) of the video between the brackets.
Preview before posting to verify it was done properly.
This: [youtube]VGEPYEvjP6k[/youtube ...plus the end bracket produces the following video: