There is no surefire answer, as it depends on a number of factors such as your region, luck, and timing. However, there have been a few methods that have worked for some players in the past.
One method is to keep an eye on the Amazon Prime Rewards section on the League of Legends website. Sometimes, there will be a banner or announcement for a Prime Rewards promotion. When this happens, you can usually click through to claim your free Prime Rewards.
Another method is to keep an eye on the official League of Legends social media channels, such as Twitter, Facebook, and Instagram. Sometimes, announcements for Prime Rewards promotions will be made on these channels.
Finally, you can try contacting customer support for League of Legends and asking if there are any current promotions for Amazon Prime Rewards.