Note: The product does not work on RU, BR, CN Battle.net accounts.
Step 1: Go to Overwatch Official. (If you don't have an account, at the top of the page, click "Create Account").
Step 2: Go to the account page. Click "Add Game".
Step 3: Key in your game's cd-key and click "ACTIVATE GAME".
Step 4: Run the Blizzard Downloader to download the game and enjoy your game.
You may purchase it here, Overwatch (Global).
Visit the SEAGM Support site for quick answers and guides.