It is possible to use camera or desktop as a source for a live event. 

Once a live event is created, there is a new tab called "Source":

When the live event is ready to accept ingests, you can choose which source will be the primary one - camera or screen.

When you choose Screen as a "Picture in picture" source you can then select to stream the whole desktop, a particular window or Chrome/Edge tab:

You can then position the "Picture in picture" source in various places of the mixed stream:

If you choose to have the desktop/window as a primary source, you will be able to choose two additional presets that show clipped camera stream and full desktop/window:

Streaming using this option is currently limited to Chrome and Edge browsers. Also, only the microphone is being recorded, not the desktop sounds.