License Activation Center includes a command line utility, lacutil, that lets you activate and save a license to a file.
The only argument that is required for lacutil is the activation key. The host ID of the current machine will be used automatically, if the host ID is required. There is also an option to print out host IDs of the machine and eventually specify them manually, which can let you activate a license locked to a different machine than the current one.
Example usage:
lacutil.exe -activate -act_key X34UT-0B2VE-1LAT9-Y3S45
lacutil.exe -activate -act_key X34UT-0B2VE-1LAT9-Y3S45 -hostid ETHERNET=DABA9F60077CBD89,BIOS=Y2DVL2
lacutil.exe -hostid