If you go to Dashboard > Front End PM PRO > All Messages then take cursor to message title you can see message id.
If you do not need explicitly add message id to the subject, i prefer not to add id to the subject. We can not get id before message save. So we have to add id after message save that will save every message twice which will stress your server.
After that also if you want to add id to message subject then let me know. I will give you code to achieve that.