How do I add a full-screen button to mp4 videos?

I am making a multi-screen HTML 5 help project.  We want to add videos.  Right now the videos are mp4, but really we could convert them to anything.  I want the videos to appear on the page at a nice small specified size.  That I can do.  However, I want the user to be able to expand the video to a boarderless full-screen size, similar to Youtube videos.  I can't find any option to add a full-screen button to the video player.  Can it be done in Robohelp?  Do I have to buy an external video player?

Thanks.