You will able to receive Twitch Drops by achieving a goal when watching an Eternal Return broadcast that provides Drops.
You can check the Drop goals and progress by clicking [Options → Services → Twitch Account → Connect].
[Eternal Return Twitch Drops Page]
To learn more about how Twitch Drops work check out the link below.
[How to Discover and Claim Drops on Twitch]
Once you reach 100%, you will either see a Pop-up on Twitch notifying you, or [Profile→DROPS→Inventory] on the official Twitch homepage.
For the acquired Twitch Drops, you need to click the [Receive Now] button within the validity period in the [Inventory]. Open Eternal Return and go to [Store → Drops] to receive your rewards.