The interface com.openexchange.groupware.notify.hostname.Hostnam eService is extended by an additional method with the following signature:

/**
* Returns the host name part used in generated links to internal objects for guest user accounts; meaning the replacement for
* [hostname] in URL template defined by property object_link in properties file
* notification.properties. Additionally this service may be used for the host name when generating direct links into
* the UI.
*
* @param userId The user ID or a value less than/equal to zero if not available
* @param contextId The context ID or a value less than/equal to zero if not available
* @return The host name part used in generated links to internal objects or <code>null</code> (if user ID and/or context ID could not
* be resolved or any error occurred).
*/
String getGuestHostname(int userId, int contextId);

Further details about the guest hostname are available at https://oxpedia.org/wiki/index.php?title=AppSuite:Sharing_and_Guest_Mode#Gu est_Hostname .