OptionProvidor overruled

The optionProvider on Client Credentials solves this issue: #3157501: Add Scopes to ClientCredentials Grant Type → . Rather than using the upstream default class, you should extend \Drupal\oauth2_client\OAuth2\Client\OptionProvider\ClientCredentialsOptionProvider in your own custom class and I think you can copy right over the code from \League\OAuth2\Client\OptionProvider\HttpBasicAuthOptionProvider::getAccessTokenOptions as it adds what it does onto the options configured by the parent. Your extension will pass the check in the code that you have removed.

I've created our own custom class as you suggested, but how can i add the scopes to the request.
My optionprovider is created in the Oauth2ClientPluginBase.php at line 211 => $collaboratorObjects[$type] = new $collaborator();
I can't use tesame method like the ClientCredentials option providor => $provider->setOptionProvider(new ClientCredentialsOptionProvider($clientPlugin));
It has a parameter and the custom optionsprovider not.

I wouldn't set this option provider via the annotation method (which is used at Oauth2ClientPluginBase:211 as you note). I would leave that off and in your plugin, override and extend Oauth2ClientPluginBase::getProvider.

Assuming:

class SomeServiceAuthOptionProvider extends ClientCredentialsOptionProvider {

  public function getAccessTokenOptions($method, array $params) {
    // copied from \League\OAuth2\Client\OptionProvider\HttpBasicAuthOptionProvider
    if (empty($params['client_id']) || empty($params['client_secret'])) {
          throw new InvalidArgumentException('clientId and clientSecret are required for http basic auth');
     }

     $encodedCredentials = base64_encode(sprintf('%s:%s', $params['client_id'], $params['client_secret']));
      unset($params['client_id'], $params['client_secret']);

      $options = parent::getAccessTokenOptions($method, $params);
      $options['headers']['Authorization'] = 'Basic ' . $encodedCredentials;

      return $options;
  }

}

In your plugin, the override would be

  public function getProvider(): GenericProvider {
    $provider = parent::getProvider();
    $client = $this->getClient();

    $provider->setOptionProvider(new SomeServiceAuthOptionProvider($clientPlugin));
    return $provider;
  }

Then the provider will be set when the code flow gets to ClientCredentials::getAccessToken and it will be an instance of ClientCredentialsOptionProvider

Thank you very much for your response, it solved the issue.
Here are my changes for others who has tesame issues
My custom optionprovider :

class UcllHttpBasicAuthOptionProvider extends ClientCredentialsOptionProvider {

  /**
   * A string of scopes imploded from the Oauth2ClientPlugin.
   */
  private string $scopeOption;

  public function __construct(Oauth2ClientPluginInterface $clientPlugin) {
    $scopes = $clientPlugin->getScopes();
    if (!empty($scopes)) {
      $this->scopeOption = implode($clientPlugin->getScopeSeparator(), $scopes);
    }
  }
  public function getAccessTokenOptions($method, array $params) {
    if (empty($params['client_id']) || empty($params['client_secret'])) {
      throw new InvalidArgumentException('clientId and clientSecret are required for http basic auth');
    }

    $encodedCredentials = base64_encode(sprintf('%s:%s', $params['client_id'], $params['client_secret']));
    unset($params['client_id'], $params['client_secret']);

    if (!empty($this->scopeOption)) {
      $params['scope'] = $this->scopeOption;
    }

    $options = parent::getAccessTokenOptions($method, $params);
    $options['headers']['Authorization'] = 'Basic ' . $encodedCredentials;

    return $options;
  }
}

My oauth2client plugin :

/**
 * OAuth2 Client to authenticate with our services
 *
 * The client_id and client_secret is set through /admin/config/system/oauth2-client
 *
 * @Oauth2Client(
 *   id = "<your-client-id>",
 *   name = @Translation("<YOUR CLIENT NAME>"),
 *   grant_type = "client_credentials",
 *   token_uri = "https://<your-domain>/oauth2/token",
 *   authorization_uri = "",
 *   resource_owner_uri = "",
 *   scopes = { "<your-scopes>" },
 * )
 */
class UcllApi extends Oauth2ClientPluginBase {

  use StateTokenStorage;

  /**
   * {@inheritdoc}
   */
  public function getProvider(): GenericProvider {
    $provider = parent::getProvider();
    $provider->setOptionProvider(new UcllHttpBasicAuthOptionProvider($this));
    return $provider;
  }

  /**
   * {@inheritdoc}
   */
  public function getRedirectUri(): string {
    return "";
  }
}

Happy to help and glad it worked out!