In order to receive the RDS CAL product key for your Windows Server, you need to send us your License Server ID first. What is License Server ID?
The License Server ID (LSID) is a unique code generated for the respective Windows Server after RDS roles have been installed and run on it. RDS CALs can then be generated based on that LSID. RDS CALs generated for a given LSID can only be used for that specific server. If the LSID changes (due to changes or reactivation of the server, for example), new RDS CALs need to be generated.
Open RD Licensing Manager: click Start → Administrative Tools → Remote Desktop Services → Remote Desktop Licensing Manager → Select the RDS server → right-click and select Properties (the LSID is available in the last row, below the Product ID).