As of now (Nov '24), the redirect URIs can be set in the new AWS Congnito UI by doing the following:

1. Navigate to: Amazon Cognito > User pools > [ your_user_pool ] > App clients > App client: [ your_client ]
2. Choose the Login pages tab on the overview page
3. Choose Edit in the Managed login pages configuration pan
4. Add/remove your redirect URIs as desired (I added a localhost:3000 during development but don't intend to leave it there for prod)
5. Save and you're done.