SharePoint: Add existing field to content type using PowerShell

This PowerShell Script is used for add existing site field or column to a content type:   cls
Remove-PSSnapin Microsoft.SharePoint.Powershell -ErrorAction SilentlyContinue
Add-PSSnapin Microsoft.SharePoint.Powershell

$site = Get-SPSite -Identity "Your Site Url"
$web = $site.RootWeb

$ct=$web.ContentTypes["ContentType"];
$fieldAdd=$web.Fields["Field Name"]
$fieldLink=New-Object Microsoft.SharePoint.SPFieldLink($fieldAdd)
$ct.FieldLinks.Add($fieldLink);
$ct.Update()

$web.Dispose()
$site.Dispose()