Eurail can't book seats for domestic trains in the UK, and that's why they say "Not available from Eurail.com”. They can be booked on the GWR website. See this page for instructions:
https://www.seat61.com/interrail-and-eurail-reservations.htm
You'll also see where and how to book seats on other trains. This is the best guide to pass reservations and it's usually best not to use the Eurail website for reservations.
Seat reservations don't need to be added anywhere. They're separate from the pass.
For first time Eurail travellers, this page may also be interesting:
https://www.seat61.com/how-to-use-a-eurail-pass.htm