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Thanks Scarlet7.

1) It works.
2) I don't want to make the target program (NotePad in this example) focused , so we can use the PostMessage without SetForegroundWindow(hTargetWnd) and...

Thanks, that may work.

I tried to work with PostMessage and the messages are indeed, received by the notepad.
But I found that in this case, I must know what the target program is designed to...

In order to send keystorkes or mouse clicks to other application that is not in focus, I found
in the internet the concept of using SendInput while attaching my application thread to the...