Valid values are:
During Client Registration, the Relying Party (OAuth Client) MAY register a Client Authentication method. If no method is registered, the default method is client_secret_basic.
Other Authentication Methods may be defined by extension. If unspecified or omitted, the default is client_secret_basic, denoting HTTP Basic Authentication Scheme as specified in RFC 6749 Section 2.3.1 of OAuth 2.0.