you simply reference the CustomerID from your main form in the recordsource of the pop-up form (Forms!frmYourMainFormWithSubform!CustomerID) then in the OnCurrent event of the main form you check to see if the pop-up form is open (using the public function below) and if it is you simply requery it.
Code:
Public Function IsOpen(ByVal strFormName As String) As Boolean
IsOpen = False
' is form open?
If SysCmd(acSysCmdGetObjectState, acForm, strFormName) <> 0 Then
' if so make sure its not in design view
If Forms(strFormName).CurrentView <> 0 Then
IsOpen = True
End If
End If
Exit Function
End Function
Cheers,
Vlad