Using the src attribute on the scene tag, XSeen will load the specified file from the URL into the XSeen display. It loads after any inline content and is automatically parsed as soon as the content finishes loading. The external file format is XML with a root tag of x-scene. All attributes on the x-scene tag in the external file are ignored. This is documented in the scene tag.
The working example is available. This example uses a model from NIH’s 3D Print Exchange and converted from X3D.