When commissioning your Droplet through Strormcloud application, you are requested to scan the QR code on the Droplet, unless you prefer to manually enter the serial number. 

This serial number is what we call the Droplet ID and is located below the bar code on the Droplet.

If you cannot reach the QR code or serial number on the Droplet because of the way it's been mounted,  it can also be found on the packaging it arrived in.