To broadcast Stream Avatars as an overlay on your livestream, you need to add it as a visual source. The visual source that you need depends on your broadcasting software of choice.
If your broadcaster software is not shown here it will probably still work, however, some broadcasting software lack features so there may be some compromises with the type of visual source you add.
Stream Avatars acts as a Game, so if you can, capture it as you would if you were streaming a Game. Feature rich broadcasters have options such as "Game Capture" and "Allow Transparency". Otherwise you might need to use a "Window Capture".
Choosing the Layer
To ensure stream avatars is displaying on top of stream sources, you can adjust the order of the sources. The closer a source is to the top, the higher it's layer is. That will allow Stream Avatars to be on top of other sources, like your primary stream content (Video Games, Work, Art, Webcam).
click and drag from the bottom to the top
stream avatars on top of everything
No Game-Capture feature?
If you don't have a game-capture or similar feature, you can use a regular window capture and add filter effects to take out the background color of Stream Avatars (effectively making it an overlay).
How to match the Stream Avatars background to the Color Key's color:
Navigate to General > Window Settings, and click on the "Background Color" box.