Spring-Security-Oauth2: Full authentication is required to access this resource

The client_id and client_secret, by default, should go in the Authorization header, not the form-urlencoded body.

1. Concatenate your client_id and client_secret, with a colon between them: abc@gmail.com:12345678.
2. Base 64 encode the result: YWJjQGdtYWlsLmNvbToxMjM0NTY3OA==
3. Set the Authorization header: Authorization: Basic YWJjQGdtYWlsLmNvbToxMjM0NTY3OA==