Insert Content into Text File in Powershell

Just output the extra text e.g.

(Get-Content $fileName) | 
    Foreach-Object {
        $_ # send the current line to output
        if ($_ -match "pattern") 
        {
            #Add Lines after the selected pattern 
            "Text To Add"
        }
    } | Set-Content $fileName

You may not need the extra “n` since PowerShell will line terminate each string for you.