I don´t know any way to enter the PIN in the Client automatically, because it would be a security risk.

If you want to do it anyway it i would use an automation tool like AutoIT

AutoIt v3 - Downloads

With this tool you can compile executable files to run on your PCs.

Here is a little Script that would do that. It activates the Astaro Client Windows, then presses alt-n, then c (for connect), then it waits for the pin window to pop up, sends the pin 1234 and presses the ok button.

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; Select Client
WinActivate("Astaro Secure Client")
WinWaitActive("Astaro Secure Client")

; PIN
Send("!n")
sleep(500)
Send("c")
sleep(50)
WinWaitActive("Enter PIN")
Send("1234")
sleep(50)
Send("!o")