There are a few possible ways to add Sky Go Extra to your account, firstly there is this link:
https://www.sky.com/shop/tv/sky-go/
Secondly if you are a Sky VIP customer as you have activated the VIP program via the MySky app and your level is Gold or above then you can claim your free sky go extra reward via the app.
Thirdly yopu can call Sky and they will tell you what the best option is for you to get it on your account.