PowerShell - Close running script with another script
You could add a line to FORM.ps1 to export its $PID to a temporary file then use then import the $PID into your STOP.ps1 so you can kill the task.
for FORM.ps1 add
$PID | Out-File -FilePath $env:USERPROFILE\AppData\Local\Form.tmp
for STOP.ps1 use
$ErrorActionPreference = 'SilentlyContinue'
Unregister-ScheduledTask -TaskName "TaskName" -Confirm:$false
$FormPID = Get-Content $env:USERPROFILE\AppData\Local\Form.tmp
taskkill /pid $FormPID