You can earn Fortnite Drops in the official broadcast or by watching any Drop-supported Twitch channel.
You'll need to make sure your Twitch account is connected to your Epic Games Account prior to the event. You can do this in your Connections settings on your Epic Games Account.
Note: Players with Cabined Accounts will be unable to earn Twitch Drop rewards until a parent/guardian completes account setup. However, depending upon your age, some features may not be available until you are older. For example, you may not be able to link to services whose rules don’t allow users your age. For more information visit our FAQ.
Once an approved channel is viewed for the required time for the current Twitch Drop event, a notification will appear on your Twitch account confirming you have met the eligibility requirements. You can then navigate to the Twitch inventory section to claim the content.
Be sure to claim it in the time frame stated in the notification.