How to customize the Authorization header of the OAuth2 token request

Suppose you have the following configuration:

spring:
  security:
    oauth2:
      client:
        registration:
          test-client:
            provider: spring
            client-id: aladdin
            client-secret: "open sesame"
            authorization-grant-type: client_credentials
            redirect-uri: "{baseUrl}/login/oauth2/code/{registrationId}"
            scope: resource:read
        provider:
          spring:
            authorization-uri: http://auth-server:9000/oauth/authorize
            token-uri: http://auth-server:9000/oauth/token

In the example from the OP, the OAuth2AuthorizedClientManager supports client_credentials grant to make an access token request. This would be useful for ex. if we want to implement the following (fictional) endpoint:

@RestController
public class TokenController {
    @GetMapping("/token")
    public OAuth2AccessToken token(@RegisteredOAuth2AuthorizedClient("test-client") OAuth2AuthorizedClient testClient) {
        return testClient.getAccessToken();
    }
}

The OAuth2AuthorizedClientManager provides the OAuth2AuthorizedClient that is injected in this example. It can be configured with a custom converter as follows:

Note: The following is an expanded example adapted from the Client Credentials section of the reference documentation.

    @Bean
    public OAuth2AuthorizedClientManager authorizedClientManager(
            ClientRegistrationRepository clientRegistrationRepository,
            OAuth2AuthorizedClientRepository authorizedClientRepository) {
        // @formatter:off
        OAuth2AuthorizedClientProvider authorizedClientProvider =
            OAuth2AuthorizedClientProviderBuilder.builder()
                .clientCredentials((builder) ->
                    builder.accessTokenResponseClient(clientCredentialsAccessTokenResponseClient())
                        .build())
                .build();

        DefaultOAuth2AuthorizedClientManager authorizedClientManager =
            new DefaultOAuth2AuthorizedClientManager(
                clientRegistrationRepository, authorizedClientRepository);
        authorizedClientManager.setAuthorizedClientProvider(authorizedClientProvider);
        // @formatter:on

        return authorizedClientManager;
    }

    @Bean
    public OAuth2AccessTokenResponseClient<OAuth2ClientCredentialsGrantRequest> clientCredentialsAccessTokenResponseClient() {
        // @formatter:off
        OAuth2ClientCredentialsGrantRequestEntityConverter requestEntityConverter =
            new OAuth2ClientCredentialsGrantRequestEntityConverter();
        requestEntityConverter.setHeadersConverter(headersConverter());

        DefaultClientCredentialsTokenResponseClient accessTokenResponseClient =
            new DefaultClientCredentialsTokenResponseClient();
        accessTokenResponseClient.setRequestEntityConverter(requestEntityConverter);
        // @formatter:on

        return accessTokenResponseClient;
    }

    private static <T extends AbstractOAuth2AuthorizationGrantRequest> Converter<T, HttpHeaders> headersConverter() {
        // @formatter:off
        Converter<T, ClientRegistration> clientRegistrationConverter =
            AbstractOAuth2AuthorizationGrantRequest::getClientRegistration;
        return clientRegistrationConverter
            .andThen((clientRegistration) -> {
                HttpHeaders headers = new HttpHeaders();
                headers.setAccept(Collections.singletonList(MediaType.APPLICATION_JSON_UTF8));
                headers.setContentType(MediaType.valueOf(MediaType.APPLICATION_FORM_URLENCODED_VALUE + ";charset=UTF-8"));
                headers.setBasicAuth(clientRegistration.getClientId(), clientRegistration.getClientSecret());
                return headers;
            });
        // @formatter:on
    }

If we wanted to support the authorization_code grant for logging into the application, we could use the following configuration (with the above methods and @Bean definitions contained within):

@EnableWebSecurity
public class SecurityConfiguration {

    @Bean
    public SecurityFilterChain securityFilterChain(HttpSecurity http) throws Exception {
        // @formatter:off
        http
            .authorizeRequests(authorizeRequests -> authorizeRequests
                .anyRequest().authenticated())
            .oauth2Login(oauth2Login -> oauth2Login
                .tokenEndpoint(tokenEndpoint -> tokenEndpoint
                    .accessTokenResponseClient(authorizationCodeAccessTokenResponseClient())));
        // @formatter:on

        return http.build();        
    }

    @Bean
    public OAuth2AccessTokenResponseClient<OAuth2AuthorizationCodeGrantRequest> authorizationCodeAccessTokenResponseClient() {
        // @formatter:off
        OAuth2AuthorizationCodeGrantRequestEntityConverter requestEntityConverter =
            new OAuth2AuthorizationCodeGrantRequestEntityConverter();
        requestEntityConverter.setHeadersConverter(headersConverter());

        DefaultAuthorizationCodeTokenResponseClient accessTokenResponseClient =
            new DefaultAuthorizationCodeTokenResponseClient();
        accessTokenResponseClient.setRequestEntityConverter(requestEntityConverter);
        // @formatter:on

        return accessTokenResponseClient;
    }

    // ...

}