Any javascript api is intentionally made this way I guess. Just imagine if you go to a website and it automatically turns on in fullscreen without you doing anything at all. You would be exposed to a whole world of annoying popups and other stuff you dint even intend to open. So any such script requires user interaction to work. Switching to fullsreen without user interaction quite very much feels like malicious intent. Example if a site such as this opened up in fullscreen : Virtual Desktop someone may pretty much be confused of what just happened to their desktop.
If you still want to try a partial workaround maybe then take a look at following answer :
Partial WorkAround