This is a conceptualization issue. First, this isn't an autonumber, lets call it a 'custom id'. To the database, because this is two distinct pieces of data (month and autonumber), you should set it up that way. That means whenever people need to see it you create this custom id by merging those two fields of your table and presenting it.
For example, if TableID was the autonumber field name and MonthData was the month field name, you could display this custom id by doing this:
=MonthData & "-" & TableID