You can achieve this by adding in your script a field (as MonthYear) that combines both of Year and Month together like this:
Month & '-' & Year as MonthYear,
Or if you have the actual date you can do this:
Year(TrxDate) as Year,
Month(TrxDate) as Month,
Date(MonthStart(TrxDate), 'MMM-YYYY') as MonthYear
Then simply use that MonthYear field as a listbox.
Hope this helps.