There is a breather on the top of the gearbox, you need to fill it through that using a long thin piece of hose with a funnel in the end, when you fill it use 2.7 litres, that was the last revised recommendation from fiat on quantity.
The breather just pulls up for the lid of it to come off, that's also where it gets in.
Not sure if they changed it when the updated the engine or not.
Thats great, thanks for replies! And omg what a stupid design re re filling the box with oil 🙈