Get-UserSid.ps1

<#
.SYNOPSIS
    Returns the SID for the specified credentials

.DESCRIPTION
    Returns the SID for a user name, using System.Security.Principal.NTAccount. If
    no UserName is specified, then the current user name is used.

.NOTES
    History:
    Date        Author      Description
    2019-06-24  rjvs        Initial version
#>

[CmdletBinding()]
param(
    [string]$UserName,         # Optional: User Name (may include domain, if $UserDomain is null/empty)
    [string]$UserDomain        # Optional: User Domain
)

if ([string]::IsNullOrEmpty($UserName)) {
    Write-Verbose "`$UserName is null or empty"
    $UserName = [System.Security.Principal.WindowsIdentity]::GetCurrent().Name
    Write-Verbose "Current UserName: $UserName"
}
if ($UserName.IndexOf('\') -ge 0) {
    $parts = $UserName.Split('\')
    $UserDomain = $parts[0]
    $UserName = $parts[1]
}

Write-Verbose "UserName: $UserName"
Write-Verbose "UserDomain: $UserDomain"
$user = New-Object System.Security.Principal.NTAccount($UserDomain, $UserName) 
$sid = $user.Translate([System.Security.Principal.SecurityIdentifier]) 
@{ UserName = $UserName; UserDomain = $UserDomain; Sid = $sid.Value }