I figured out how to create an Internal Link. It works OK, but the implementation is a little clumsy.
After you copy an item, you can right-click on the canvas and there’s an option to “Paste as a Link”. This option only appears in the context menu, not in the main menu. Also, the Paste as Link option only appears when you’re pasting onto the canvas, not when you’re pasting into a note.
Here’s how I would improve this feature:
Add Paste as Link to the main menu
Add a toolbar icon to paste as a link
Allow for pasting as a link into notes.
Create a new “Duplicate as Link” option that would create a duplicate of the selected Topic, linked back to the original.