Naturally, there is no law requiring that such an alarm be fitted.
Naturally, there is no law requiring that such an alarm be fitted ... so it is quite legal to disable it.

But it is a real nuisance. Although l wear my seat belt on the road, l have no intention of putting it on to drive the hundred yards to the gate. But off goes the alarm. Or if l just drive down the paddock to feed the cows ... off goes the alarm.

You can not it is against the law.

l am not sure if this will work for a Forester, but it did for my Baja.

Turn the key to on. Fasten/unfasten the seat belt as quickly as possible about 20 times. Start the car.

l still get the initial ding when l start up, but the door ajar and seat belt chimes are disabled. l hope this helps.

Why would you want to? You could ask the dealer if it is a programming option which is legal. Or you can wear a seat belt and things will be fine.