Hello @Eric Huang (M365) ,
Thanks for the question and using MS Q&A platform.
As per the discussion on the below threads, it seems like it is not yet supported in Synapse Notebooks.
Stakcoverflow thread: https://stackoverflow.com/questions/67883213/consuming-nuget-packages-in-azure-synapse-c-sharp-notebooks-from-nuget-feed-in-a
MS Q&A thread: https://learn.microsoft.com/en-us/answers/questions/426375/how-to-consume-nuget-package-in-azure-synapse-note.html
Here is an exiting feedback on the same feature request logged by a different user in IDEAS forum: https://feedback.azure.com/d365community/idea/46ede6a9-0a25-ec11-b6e6-000d3a4f07b8. I would recommend you to please up-vote and/or comment on it which would help increase the priority of the feature request suggestion .
I will also reach out to relevant product owners to see if there is any workaround or any other update on this request and get back to as soon as I have an update.
Thank you