I just did the default dictionary entry using cup/Cup as described earlier, and made sure to make the context "Whole Word" at the end of the dialog, as I don't want, say, "cupid" mispronounced, and it definitely works. I used the webpage you reference for testing.
Brian - Windows 10 Pro, 64-Bit, Version 1903, Build 18362
The color of truth is grey.
~ André Gide