Depends from Hostel to Hostel on some you have to fill at their own booking website a code like Eurail or Interrail on others they have special codes :)

Interrail Pass Benefits
Here the new combined (Interrail/Eurail) PassBenefit Platform

thank you so much. i e-mailed them to figure it out.

Who you emailed too? Eurail or the Hostel?

And for which hostel´s have you looked?

Each Hostel have a own process how you can use the Interrail/Eurail Discount


on most of them you have to book directly at the own page of the Hostel and just add the discount Eurail or Interrail in the Cupon/Discount section within the booking process :)

add few Hostels you can just show your Railpass and get the discount directly at the Check in :)

in the relevant pass benefits section of each country you find the Hostels that offer discounts and the process how to do it :)

A&O Hostels offer a own booking process for Railpass user