When editing the marathon, I don't see anywhere to enter the twitch account

https://gyazo.com/6c7941d2b5ffd3dd04651a4c85d27231

Setting the twitch account can be done on the edit schedule page rather than the edit marathon page.

It looks a bit weird, isn't it ? I mean shouldn't all networks be at the same place ?

Presumably it's implemented the way it is because it was designed with ESA's dual stream setup in mind, where the two schedules each have their own twitch link.