How to reserve Nanki

Hi,

I would like to travel from Shingu via Nagoya to Tokyo using Nanki limited express train and Hikari train using my JR pass.

I read a lot online about the Nanki train requiring some sort of additional ticket/reservartation/limited express surcharge, even if you own a JR pass.

But I wonder: where can I buy the needed limited express surcharge ticket? Is it possible online? Does the extra ticket include a seat reservation or not?

Please help with my confusion.

Thanks.