msiexec /i location_of_msi_file LICENSEKEY="key" /qb+
where key is the license key. Be sure to enter the numbers separated with a hyphen (-) (for example, ab123-c4d5e-678fg) as you do when entering the license key through the user interface.
For example, to run an unattended installation from D: and install a Kerberos license, you might enter:
msiexec /i d:\ViewNowX\X9.msi LICENSEKEY="00C0-ADCDE-D123" /qb+