Q:

How to add the new unity uxml(via ui builder) to game screen?

I installed the new Unity UI Builder. Everything works just fine...
But how do I add the uxml to my scene or how do I attache it to camera? 
I tried to drag and drop the uxml to camera and I searched for a component, but I couldn't find anything... Is there any official documentation?

A:

You must use an gameObject with PanelRenderer.cs attached to it:

For now, I only know you have to set UXML file and USS file. In play mode, by magic (I still don't understand how unity does it), it will render.
In the Github project you can see how things are for now. UIElementsUniteCPH2019RuntimeDemo 
EDIT:
The project linked above uses UI Builder 0.8.

A:

Runtime support is still not available using the packet manager.
You can add the preview version to your project by adding the following line to your manifest.json (it's located in the projects Packages-folder)
"com.unity.ui.runtime": "0.0.4-preview"

You may want to check whether a newer version is available.
Keep in mind that the preview version is intended for previews only. The api and workflow might change with newer versions.
Once runtime support is enabled you can use the Panel Renderer as described above.

