Once you've linked a Twitch account to your MLB The Show account, you'll be eligible to earn drops on future watched streams.
Any earned drops will need to be claimed on Twitch, which you can learn about by clicking here.
Claimed drops will show up in the Packs section within Diamond Dynasty.
Please be aware that it may take 24-48 hours for earned Twitch Drops to appear after they've been claimed.
Other Useful Twitch FAQs:
Creating an MLB The Show Account