$strScriptUser = "<Domain>.local\<Account>"
$strPass = "<Password>"
$PSS = ConvertTo-SecureString $strPass -AsPlainText -Force
$cred = new-object system.management.automation.PSCredential $strScriptUser,$PSS
$Now = Get-Date -Format g
$title = 'ADD TITLE FOR POPUP'
$msg   = 'ADD MESSAGE FOR POPUP'
$text = [Microsoft.VisualBasic.Interaction]::InputBox($msg, $title)
$Log = $env:username + ", " + $env:computername + ", " + $Now + ", " + $text

Invoke-Command -ComputerName "SYDSRV02" -scriptblock {
Stop-Service -name "Test Service"
Start-Sleep -s 25
Start-Service -name "Test Service"
} -Credential $cred
Add-Content -Path \\<Server>\Path\Textfile.txt $Log