https://wccftech.com/12-months-nintendo-switch-online/

All Twitch Prime subscribers have the chance of getting up to 12 free months of Nintendo Switch online, and they’re incredibly easy to redeem.

All Twitch Prime and Amazon Prime members simply have to log in into their Twitch Prime account and hit the Get 3 Months of Nintendo Switch Online” box. You will be asked to sign in to your Nintendo account to claim your reward. The additional 9 months are available to all those who have been Twitch Prime members for at least 60 days. As the offer is going to be valid until September, you can become a Twitch Prime member right now and get the full benefits of the promotion later on.