I don't know about that specific listing from your link, but I dropped a pin right on l'avenue Gustave-Eiffel behind the tower and got the code V75W+43 Paris, France. Plus codes appear right under the address line.
If you're on mobile, you can get the plus code for any location by just holding your finger on the map, and then expanding the card that appears. It should include the lat/lng and plus code.
The eiffel tower search result is the whole park, which is larger than the area spanned by a single plus code and therefore does not display it's plus code.
