It is necessary that the plugin extension for the custom protocol implements the URLStreamHandlerPluginExtension interface. Without it, you cannot use your plugin, because Oxygen XML Author is not able to find the protocol handler.
You can choose also to implement the URLChooserPluginExtension interface. It allows you to write and display your own customized dialog for selecting resources that are loaded with the custom protocol.
An implementation of the extension URLHandlerReadOnlyCheckerExtension allows you to:
It is useful when opening and editing CMS resources.