If you use/have PowerShell built into your WinPE image, you could avoid using any files at all with this one-liner in the "Prestart command settings">"Command line" box:
powershell.exe -noprofile -command "$TSEnv = New-Object -ComObject Microsoft.SMS.TSEnvironment;$TSDeploymentID = Read-Host 'Enter the TS Deployment ID: ';$TSEnv.Value('SMSTSPreferredAdvertID') = $TSDeploymentID"
Partly leaving this here so I can find it in future. ?