this.form = this.fb.group({
        'start_dates' : this.fb.array([]),
         ...
    });

startDates.push(this.fb.group({
            city : startDate.city,
            city_name : startDate.city_name,
            date : startDate.date,
            enabled : startDate.enabled
        }));
this.form.controls['start_dates'] = startDates;

let control = this.form.get('start_dates').setControl(index, start_date);

let control = <FormArray>this.form.get('start_dates');
control.setControl(index, event);