You’ll be able to earn five League Tokens for every hour watched of a live Overwatch League Match. Note, you must watch these matches on the official site, or through the official Overwatch League App. The time you’ve viewed these matches will be tracked. For example, if you watch 30 minutes, take a break, and then come back later and watch another 30 minutes, you’ll be credited for one hour total. You can only accumulate time during live matches.
If you want to redeem your token, you need to have a valid license for the game. If you’re on console (Xbox One, PS4, Switch), you need to link your respective Xbox Live, PS Plus, or Nintendo accounts to Blizzard. Note that any token can take up to 48 hours before they’re delivered to you.
The official post then lists several countries with which your Blizzard account can be associated in order to earn tokens.