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You’ll need to add a new schedule for this from the Settings -> Cron Schedules screen, then you can use that schedule when you edit a cron event.
That said, an event with such a short interval may not be reliable, depending on how much traffic your website gets. More info here: https://github.com/johnbillion/wp-crontrol/wiki/Cron-events-that-have-missed-their-schedule .