Install openssh on your unraid (unmenu can do it for you)
For windows, get plink.exe and put it anywhere (i did it on my desktop)
Create a bat file (named sleepunraid.bat or whatever) witch content is:
cd C:\pathtowhereplinkis
plink.exe -ssh 192.168.xxx.xxx -l root -pw unraidpassword "echo 3 > /proc/acpi/sleep" ; \
If your unraid is not password protected, then it could also work with telnet (no ssh and no plink needed)
telnet 192.168.xxx.xxx -l root "echo 3 > /proc/acpi/sleep" ; \
or just open a telnet session and pass the command.