How to get the Users' Eikon Messager ID?

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JET cannot supported it officially, From Kolba, Nicholas (Financial&Risk): Messenger does use the JET pub/sub message bus for all of its communication. So, technically, an App can access this through JET, however, there is no officially supported API to do so. You would need to work with the messenger group directly to work out doing this. Going forward, we will be working on supporting this as part of the JET API (with proper security/permissioning controls). Also, please note that 3rd party Apps are currently blocked from accessing any of the EM pub/sub messages, so the above would not work in the case of an external App.