I agree that it is likely something do to with the keys, but I’ve tried with the “All users” and “Single user” settings (using an admin username) and still I get the not found error you posted.
The key is correct, the URL is correct and I’ve tried all the different settings as well as other endpoints listed in the api examples. All return “The requested URL or resource could not be found”. I can’t think of much else to do 
I’m guessing the user scopes do not apply here for global keys?