The simple answer is you can't!
Currently, Nabble serves an HTML4 page and HTML4 has no mechanism for playing movie files. That's why Nabble ask you to upload your file to YouTube, Vimeo or LiveLeak and embed the link in your Nabble forum. Those sites both incorporate a player and provide sufficient storage space for the files (which tend to be much larger than still images).
The w3schools code will not work as that assumes you are creating an HTML5 page. I recall that Nabble have a plan to move to HTML5 at some point, but you still shouldn't assume that that will imply direct insertion of video will be permitted because of the extra storage requirements that would imply.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.