You can link a Named Destination (even in another file) directly, without using a URL (which is not a good idea anyway, since it won't work if the files are moved to another folder, for example). To do so create a Link and set its action to "Go to a page view". A floating window will open with a "Set Action" button. Switch to the other file, open the Destinations panel and double-click the Destination you want the link to point it, and it will do so. The floating window will disappear and if you examine the properties of the link you should see it says "Go to a page in another document", with the path of the file and the destination name you clicked on below it.
Note that although the path seems absolute, it is in fact relative, so if you move both files to another folder (or another computer), the link will still work.
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