Answer
This is done through an IFS Custom Event/Event Action running a PL/SQL block: use Client_SYS.Add_Info('<LU_or_context>', '<message text>'); for a simple informational message box, or Error_SYS.Record_General('<LU_or_context>', '<message text>'); to show it as an error-style message that also blocks the underlying action. There's no separate 'client control' API to write this kind of message directly — it's driven through the standard event/event-action mechanism calling these PL/SQL procedures.