Easiest way to grant "Log on as a service" to a Windows user from the command-line?

The easiest way to do this from a command line is definitely using NTRights.exe from the Windows Server 2003 Resource Toolkit.

ntrights +r SeServiceLogonRight -u jeroen -m \\%COMPUTERNAME%

I use Carbon and Chocolatey.

cinst carbon -y

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Grant-Privilege -Identity $username -Privilege SeServiceLogonRight

Based on this answer: https://stackoverflow.com/a/22155390/991267