I'm attempting to interact with a Google API that Wolfram Language doesn't yet have a service for. I think I've configured the SecuredAuthenticationKey correctly, but when I try to use it, like this:

URLRead[
 HTTPRequest["https://docs.googleapis.com/v1/documents", <|Method -> "POST"|>],
 "BodyByteArray",
 Authentication → mySecuredAuthenticationKey
]

It just fails with "SecuredAuthenticationKey::atok: Couldn't retrieve access tokens from https://oauth2.googleapis.com/token."

Can I access more information about the failure? I would guess that the server's response tells me what's wrong with my request, but I can't see it.

(I tried NetworkPacketTrace, but it was far too granular for me to track the server's response.)