When adding a license key into vCenter Server using the vSphere UI, users have the ability to provide a custom label that is then associated with that specific license key.
This is certainly more useful than the system default label which uses "License N", where N is an incrementing number.
While the majority of license management can be fully automated using the vCenter Server LicenseManager API, adding a license key with a custom label is unfourntately an operation that is currently not possible with the public API.
With that said, there is a way in which you can still automate this operation 🙂