Actually exporting as a video format isn’t implemented yet. In the current alpha version (alpha 2) you are only able to export as a sequence of png’s. You can then take those png’s and convert them to a video if needed.
An alternative solution is to do a screen recording while playing your animation.
Anyway, you will be able to export as mp4 and mov prores as soon as we get it done in one of the soon following alpha releases.