There are step-by-step instructions here

https://www.seat61.com/interrail-and-eurail-reservations.htm#How_to_make_reservation_at_the_Polish_Railways_website

It may be that you’re looking in the right place for the discount?

Thank you for ‘ralderton’ for taking the time to reply. I have now managed to book reservations on my chosen train. It seemed that for some reason that when clicking on ‘V’ or ‘CHOOSE’ the system didn’t offer any options for the train required but I managed eventually to get through this  problem by doing repeated searches. The other issue was that I needed two reservations for this train but hadn’t realised that you need to do these separately!

Some of these reservation procedures are so byzantine, and poorly-documented. If it wasn’t for the Man in Seat 61 I don't think I would have managed my last trip.

Absolutely, I found two things I needed to do to get the reservations needed. First, and I believe noted elsewhere, the fact that you can only book one reservation at a time due to the need to add each individual Pass Number. However, I also found that I couldn’t make the required second reservation and simply add it to my basket / cart as the system wouldn’t offer me an option to book a seat; I had to log out and back in again!

Very frustrating that you can’t make on-line reservations on ‘international’ trains from or to Poland. Let’s hope that they are still available when I get to a booking office.

I agree that the sheer volume of knowledge at The Man in Seat 61 is invaluable.

Thank You.

Not on the PKP website but many trains to Czechia and Austria can be booked via cd.cz or tickets.oebb.at. What train do you want to book (route, date, departure time)?

Thanks for the reply. Unfortunately the two Polish international journeys I’m looking to do are Lithuania (from Bialystok) and Slovakia (from Krakow).

Krakow - Bratislava can be booked on cd.cz. Probably also on the ZSSK website. Should be €3.

Białystok to Mockava cannot be booked online. But you should be able to book online via PKP to the last stop in Poland, Trakiszki.

Thank you very much, I’ll take a look at each of the options suggested.