Answer: The Microsoft Translator API switched the method of authentication in October 2016, and turned off the Client ID and Client Secret method on May 1, 2017. The new authentication method is using a key.
You will then copy one of the two keys you receive in your Translator Text API subscription account into the "subscription key" or similarly labeled field in your software. As an example, here the makers of the CAT tool "Fluency" explain how this works with Fluency's software.
For developers of CAT tools:
If you are a developer of CAT tools, or a developer of any tool that offers translation functions to users: The most convenient method for your users to consume translation is if the user doesn't have to sign up to the Microsoft Translator API separately. You can offer translation functions in your product under your own subscription, and pass on the consumption cost to your users. Use the Microsoft Translator Contact Us form if you have any questions.
Originally published May 17, 2017