As administrator you would want to send a message to a particular user. 

Beside using "SM02" to broadcast message, it can be accomplished with executing a

 program in SE37.

Steps:

1) Execute TCODE: SE37, enter "TH_POPUP" in the function module text box and

 click "Test/Execute" button (F8)

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2) Enter the client, user and message to be show to specific user and click "Execute" button

3) Sample of the pop-up message appear on user screen