I am using a cable to attach my keyboard to my new iMac, but the power button doesn’t light up

When Apple switched from ADB to USB, their standalone keyboards lost "power on" buttons at the same time. I remember someone giving as a reason that power buttons are not allowed by the USB specifications.

So I'm not sure what "power button" you would be referring to on a USB keyboard, assuming that the keyboard is standards-compliant.

What brand of keyboard are you referring to? Is it a USB keyboard?