unclear on how I get passaver discount for eurostar train after I purchase a rail pass
Here's a quote from the Eurostar site: "Passholder fares cannot be booked online. They can only be booked at a Eurostar Terminal or through our contact centre on 08432 186 186 or +44 (0) 1233 617 575. All tickets are sold subject to availability." If you call from the US, dial 011 (US international access code) + 44 (country code for the UK) + 1233 617 575. Leave off the zero in parentheses. It's only for calls originating in the UK. However, the cheapest advance purchase point-to-point ticket (€42.50) costs less than the passholder fare (£58 which equals €65). Book up to 120 days in advance at eurostar.com and print your own ticket.
Tim is correct, you can almost always get a better deal buying Eurostar in advance over getting the passholder discount.
...and ditto for Thalys trains, if that applies to your plans.