Select Link a Twitch account and sign in to your Twitch account
Once linked, you will now have the option to Subscribe with Prime
Step 1. Open your preferred mobile browser and go to www.twitch.tv
Step 2. Once loaded, choose to view the desktop version of the site (see below)
Step 3. Click Log in and follow the standard process
Step 4. Go to your favorite stream and click Subscribe and then select Free Subscription with Prime