OAUTH using JSON

Wondering if anyone has been able to get OAUTH working with using JSON only, current testing with Advanced Rest Client using either Header of Content-Type application/json or application/x-www-form-urlencoded, in the body  {   "scope":"NSMS"    "grant_type":"client_credentials",   "client_key":"KEY",   "client_secret":"secret", }   this is the return I get   { "error": "invalid_request", "error_description": "The request is missing a required parameter, includes unsupported parameter value (other than grant type), repeats a parameter, includes multiple credentials, utilizes more than one mechanism for authenticating the client, or is otherwise malformed." }