Using powershell, how do I grant "Log on as service" to an account?

This is how I solved it:

Based on: this article

You can download Carbon from here

First import Carbon module as follows:

Import-Module -Name $Path_To_Carbon -Global -Prefix CA

[array]$UserPrivileges = Get-CAPrivileges -Identity $UserName;
[bool]$LogOnAsAServiceprivilegeFound = $false;

if ($UserPrivileges.Length > 0)
{
    if ($UserPrivileges -contains "SeServiceLogonRight")
    {
        $LogOnAsAServiceprivilegeFound = $true;
    }
}

if ($LogOnAsAServiceprivilegeFound -eq $false)
{
    Grant-CAPrivilege -Identity $UserName "SeServiceLogonRight"
}

The Powershell script below will grant the SeServiceLogonRight on the host specified by computerName to the user specified by username (the script is an excerpt from here: https://gist.github.com/grenade/8519655):

<#
.Synopsis
  Grant logon as a service right to the defined user.
.Parameter computerName
  Defines the name of the computer where the user right should be granted.
  Default is the local computer on which the script is run.
.Parameter username
  Defines the username under which the service should run.
  Use the form: domain\username.
  Default is the user under which the script is run.
.Example
  Usage:
  .\GrantSeServiceLogonRight.ps1 -computerName hostname.domain.com -username "domain\username"
#>
param(
  [string] $computerName = ("{0}.{1}" -f $env:COMPUTERNAME.ToLower(), $env:USERDNSDOMAIN.ToLower()),
  [string] $username = ("{0}\{1}" -f $env:USERDOMAIN, $env:USERNAME)
)
Invoke-Command -ComputerName $computerName -Script {
  param([string] $username)
  $tempPath = [System.IO.Path]::GetTempPath()
  $import = Join-Path -Path $tempPath -ChildPath "import.inf"
  if(Test-Path $import) { Remove-Item -Path $import -Force }
  $export = Join-Path -Path $tempPath -ChildPath "export.inf"
  if(Test-Path $export) { Remove-Item -Path $export -Force }
  $secedt = Join-Path -Path $tempPath -ChildPath "secedt.sdb"
  if(Test-Path $secedt) { Remove-Item -Path $secedt -Force }
  try {
    Write-Host ("Granting SeServiceLogonRight to user account: {0} on host: {1}." -f $username, $computerName)
    $sid = ((New-Object System.Security.Principal.NTAccount($username)).Translate([System.Security.Principal.SecurityIdentifier])).Value
    secedit /export /cfg $export
    $sids = (Select-String $export -Pattern "SeServiceLogonRight").Line
    foreach ($line in @("[Unicode]", "Unicode=yes", "[System Access]", "[Event Audit]", "[Registry Values]", "[Version]", "signature=`"`$CHICAGO$`"", "Revision=1", "[Profile Description]", "Description=GrantLogOnAsAService security template", "[Privilege Rights]", "$sids,*$sid")){
      Add-Content $import $line
    }
    secedit /import /db $secedt /cfg $import
    secedit /configure /db $secedt
    gpupdate /force
    Remove-Item -Path $import -Force
    Remove-Item -Path $export -Force
    Remove-Item -Path $secedt -Force
  } catch {
    Write-Host ("Failed to grant SeServiceLogonRight to user account: {0} on host: {1}." -f $username, $computerName)
    $error[0]
  }
} -ArgumentList $username