You can best do it via Snälltåget’s own website https://www.snalltaget.se/en by using the promotion code INTERRAIL when booking the ticket.

The only thing is that (also due to the pandemic) you can not book a single bed in a shared compartment for the Berlin-Hamburg-Malmö-Stockholm route. You can only choose between booking a full sleeping compartment (for up to 6 people), a comfort seat or a standard seat.

Hello Tarek, thanks for your answer. I must correct me, because I want to travel with my Partner in the same compartment. Is this possible or must we book different compartments? Greetings hukarobert

Of course you can travel in the same compartment. Just add a second passenger when booking the ticket and you will be placed in the same compartment (and pay the same price as if you were traveling on your own) since it is for up to 6 people. 

Hi, 
I am using the promotion code INTERRAIL when booking the ticket, but it doesn’t seem to give any discount… I need to pay
399 SEK for a Seat + next seat free or
1999 SEK for a Private compartment

Same price with or without my Interrail pass… isn’t that strange?

@Cecile Ma  For which date did you tried?

I want to travel on Thursday 26 August but I tested it at different dates in the near and far future and I get the same result, no discount. 

The discount fares are showed here https://www.snalltaget.se/en/berlin

199sek for a seat
399 for a recliner seat
or 2299 sek for a private couchette compartment (currently they didn´t sell a single berth )

It can happen that the promotional fares of Snälltaget are currently cheaper or same price like the Interrail offer because of that there is currently no difference between Interrail or not Interrail :)