Euronight train from Basel (Switzerland) to Praha (Czech Republic)

Hi everybody, 
I plan to start my interrail from Lyon (France). 
I have a connection at Mulhouse (France) and then Basel (Switzerland), where I want to take the Euronight train to go to Praha (Czech Republic). The departure from Basel is at 9.15 pm. 
The thing is I don’t see where I can book a couchette (sleeper place) for this train online. And I do not want to arrive at Basel and to have to spend the whole night in a classic seat. 
Anyone had the same issue ? 
Many thanks in advance for your help !