The UserTrackEvent should contain a way to get the source that promoted/demoted the user. Unfortunately directly adding the Sender doesn't really work as that is not part of the API.

Also depending on whether or not this is planed for the future it might break with automatic (e.g. time based) promotion/demotion features. That could be solved by using a Source object that holds the information on whether or not it is a sender and if so the sender info or a custom sender for automatic track changes (this sounds a bit hacky though)