Please note that Microsoft CALs (Client Access Licences) do not require installation media, licence keys, or setup codes. The CALs are virtual and do not appear in the Microsoft Volume Licence Centre. You can find more information regarding CALs on the Microsoft website.
Please see this link on how to install Remote Desktop CALs (RDS) https://docs.microsoft.com/en-us/windows-server/remote/remote-desktop-services/rds-install-cals
CALs without software assurance are listed under the 'products' tab in your Microsoft 365 Admin Centre: