From an elevated prompt, run vxlicinst followed by two forward slashes as seen below
 

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To install a license key from a command line interface for Infoscale 7.0 to 7.3,

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The "vxlicinst"  cmdlet is available at C:\ProgramFiles\Veritas\Veritas Shared\VPI\\

For eg: For Infoscale 7.2, the path would be 

C:\ProgramFiles\Veritas\Veritas Shared\VPI\{F834E070-8D71-4c4b-B688-06964B88F3E8}\{7.2.00000.1}

Alternately, it can be obtained using below steps

a) Extract the Infoscale Installer and it should list a folder as Tools\vxlic_tools. It will have the below contents

b) Copy the "vxlicinst.exe" locally to the server

Finally to install the license,  simply run the command "vxlicinst", as seen below

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To install a license key from a command line interface for Infoscale 7.4 and above

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Please refer Installing license for Infoscale 7.4 and above

Note: Restart of "Veritas Enterprise Administrator Service" (vxsvc) is required for the license to take effect.