How to make app avatar in WMR main screen?

In the hololens main screen, there are some apps that show a 3D model as avatar, while mine (and some others) simply show a 2D screen that is asleep.
How can I make this app avatars? And what are the limitations?

The APIs for putting Holograms to the main space are not public, yet. I see, you already discovered this thread though: https://forums.hololens.com/discussion/comment/1350

Just wanted to leave this link here for reference.