FWIW An alternate method, on desktop, is to right-click on the spot on the map, then select forecast for this location in the little window that opens. From there, mark as a favorite as you would if you had entered the name of a location in the search.
On the app (at least on iPhone), tap on the spot on the map, then select forecast for this location at the top of the screen in the options that appear.
Additionally, a search can be performed on latitude and longitude as opposed to place name, then mark as a favorite.
