Start the benchmark from your Steam Library. Go to the Options screen and click on the button to show your license key. Your license key can also be used with the standalone versions of the benchmark, where it will unlock the Advanced Edition features. You don't have to use the Steam version, even if that is where you bought it.
Your license key works just like any other 3DMark, PCMark 10, or VRMark key with the 3dmark.com online result service. If you submit at least one result from the Advanced Edition, your license key will be added to your 3dmark.com account. You can see all of your license keys on the Settings screen on 3dmark.com.