Current versions of Windows use Powershell as the default shell, so use:
$env:NODE_ENV="production"
Per @jsalonen's answer below. If you're in CMD (which is no longer maintained), use
set NODE_ENV=production
This should be executed in the command prompt where you intend to run your Node.js application.
The above line would set the environment variable NODE_ENV for the command prompt where you execute the command.
To set environment variables globally so they persist beyond just the single command prompt, you can find the tool from System in Control Panel (or by typing 'environment' into the search box in the start menu).