grantType

This is the type of authentication the client is presenting for its access-token.
If the client is looking for a client-token, then this should be set to 'client_credentials'.
If the client is looking to swap an authorization grant for an access token, this should be setto 'authorization_code'
If the client is looking to swap a refresh_token for a new access token, this should be setto 'refresh_token'