There is no event to specify which type of ad shows for Facebook Instant Game ads.
The only option is "Show Ad".
We should have the option to show Interstitial or Reward ad like we can with the MobileAdvert plugin.
This is now available in r134.
Note the action defaults to the "Interstitial" type; existing projects that show rewarded videos will need to update the action to show the rewarded video type instead.