Note: For PC platform only.
Step 1: Visit The Elder Scrolls Official Website
Step 2: Log into The Elder Scrolls Online: Tamriel Unlimited Account
Step 3: Select Redeem Code on the My Account page
Step 4: Enter the code you have received after purchase and enjoy your game.
You may purchase it here, The Elder Scrolls Online: Tamriel Unlimited (Global).
Visit the SEAGM Support site for quick answers and guides.