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I have made an Integrated app which opens at the startup & such messages will showing under that app
Then all you need to do is set up a time with an appropriate interval (which will depend on how accurate you want: a minute? Ten minutes? On the Hour? We don't know). And each time it ticks you check the current time via DataTime.Now and compare it with the time you want the message displayed.
Then it just a case of deciding where you want the message: MessageBox is simple, but very annoying if you are working intensely; Notifications in Win10 can be smoother for the user but are also easier to miss.