You must create an account with ISPSystem, using a valid email address. Once created, you will be able to purchase a license or create a trial license.
To purchase a license, click 'Purchase' on the ISPManager Lite side of https://www.ispsystem.com/software/ispmanager/order.
You will be prompted for a Name for the license (E.g., "HostwindsVPS") and the IP address, which can be found in Cloud Control or the email(s) sent about your VPS when you installed ISPManager. There are some billing options to select, then click Pay. They accept wire transfers in the EU, Visa, MasterCard, and PayPal.
Once purchased or a trial license is created, they will give you a multi-character license key that can be entered on the License page that comes up when you log into ISPManager.