I asked
the same question about a week ago and also came up empty.
A way around it I found was to get the book you want on the desktop Wishlist to be located on the Kobo web site "Recommended for you" list. That list will be reflected on the desktop "Recommended for you" the next time you start the desktop application. Then you can add it to the desktop Wishlist.
Trouble is, there appears to only be 20 slots on the desktop "Recommended for you" list, while there are 24 slots on the Kobo web site "Recommended for you" list.
So, you have to remove some books from the Kobo web site "Recommended for you" list.
Then to get a book onto the Kobo web site "Recommended for you" list, you have to search for the book on the Kobo web site a few times so that their dopey software will put it on the list.
Crazy.