UI Builder: Editor extension authoring

To enable Editor extension authoring for the current UXML document in the UI Builder, either:

Select the Canvas by selecting the root element in the UI Builder's Hierarchy pane.
Under Document Settings enable Editor Extension Authoring.

Or

Select ⋮ from the top right of the Library pane.
Enable Editor Extension Authoring.

These options will add editor-extension-mode="True" to the root UXML node.