Reserve Seat on Shinkensen

Hello,
I have purchased the JR pass and I have received the Exchange Order on my mailing address already.

Can you advice if and how it is possible to Make Seat reservations on the Shinkansen/Bullet Trains online, Prior to arrival in Japan. I do know which Trains and times I wish to travel on during my trips in Japan using JR pass.

for e.g. I would like to go from Shin-Osaka to Hiroshima on the 10th May at 8am in the morning using Tokaido-Sanyo Shinkensen. Can i reserve seats online before my arrival on this ?

Regards,
Sunil