score:2
Simply put, you can't. Online booking is always for reserving seats, it doesn't make sense to book a non-reserved seat. To get the non-reserved seat, you have to buy the ticket yourself at the station physically, which is a basic fare ticket.
Similar to what @lambshaanxy has mentioned
Note that you can book non-reserved seats e.g. in SNCF France here - @Franck Dernoncourt