If you want to emulate a space bar press with a particular button, you can assign it to a button at

Prefs > Mapping > [Browser of choice] > [Button of choice]

by selecting

Virtual Keyboard > Space(bar)

for that button. There's no guarantee, though, that this space bar press ends up controlling an embedded web video, though. Depending on input focus, it could also end up f.ex. adding a space character to the address or search fields of the browser.

Best regards, 
Felix Schwarz