ISPManager is commercial software that requires a license from ISPSystem and a Hostwinds VPS or Dedicated Server.
When you first log into the ISPManager Lite system it will prompt you for a license and links to the ISPSystem site. You can choose to buy a new license or use an existing one.
The price for ISPManager Lite is 4 Euro (about $4.50 USD) at time of writing, but the current price can be seen on https://www.ispsystem.com/software/ispmanager/order
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 transfer 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.