It doesn’t support UI for desktop view out of the box, afaik. I tried tossing up some widgets and hacking my way around to see if I could get it to work, but no luck.
It’d be a lot easier for recording gameplay (or for making YT or twitch content) if the desktop view was more like the suggested green box. I know of a few YT’rs and streamers who are very hesitant to create content with UE4 based games because of verticle box. There was a post on reddit a few months ago that outlined a method for creating a full-screen desktop view, not sure if that is the same one listed here.