How can I wake up my PC with any key?

If you run the following command, you will see a list of USB devices and if they are enabled to wake up the system.

cat /proc/acpi/wakeup

You will see a list of items such as the the one seen below.

USB0      S3    *disabled   pci:0000:00:12.0

Once you have established which device is the keyboard, let’s say USB0, run this command.

sudo su
echo USB0 > /proc/acpi/wakeup

Now, if you run the command from the start again, you should see enabled next to the device.

USB0      S3    *enabled   pci:0000:00:12.0

That worked for me, and I can wake up my desktop with any key.

Details on the wakeup codes for devices can be found here (link courtesy of Santhana Krishnan).