Hello
Thanks for your feedback
Roundcube support the oauth2 in "standard" not via plugin.
You can find the parameters in the default.inc.php
I copy past to this parameters below with my parameters in the config.inc.php
If you can help me much appreciate
Fred

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OAuth
----------------------------------

Enable OAuth2 by defining a provider. Use 'generic' here
$config['oauth_provider'] = null;

Provider name to be displayed on the login button
$config['oauth_provider_name'] = 'Google';

Mandatory: OAuth client ID for your Roundcube installation
$config['oauth_client_id'] = null;

Mandatory: OAuth client secret
$config['oauth_client_secret'] = null;

Mandatory: URI for OAuth user authentication (redirect)
$config['oauth_auth_uri'] = null;

Mandatory: Endpoint for OAuth authentication requests (server-to-server)
$config['oauth_token_uri'] = null;

Optional: Endpoint to query user identity if not provided in auth response
$config['oauth_identity_uri'] = null;

Optional: disable SSL certificate check on HTTP requests to OAuth server
$config['oauth_verify_peer'] = true;

Mandatory: OAuth scopes to request (space-separated string)
$config['oauth_scope'] = null;

Optional: additional query parameters to send with login request (hash array)
$config['oauth_auth_parameters'] = [];

Optional: array of field names used to resolve the username within the identity information
$config['oauth_identity_fields'] = null;

Boolean: automatically redirect to OAuth login when opening Roundcube without a valid session
$config['oauth_login_redirect'] = false;