Amazon Prime subscribers, among their other perks, enjoy access to Twitch Prime, a service that offers free channel subscriptions in addition to other monthly goods. This month, Twitch has partnered with Jagex to hand out a month of free membership and a few other bonuses. Log in to Twitch between April 11th and May 8th to claim your code and redeem a month of membership, 200 RuneCoins, 15 treasure hunter keys, and a Prime Colossus Legendary Pet.
The code itself needs to be redeemed by June 9th, with certain items (Treasure Hunter keys) not available for Ironman RuneScape accounts. Check out the instructions on the Twitch page for more details.