You can reserve via the website of Austrian Rail oebb.at By following this manual

Thanks, that was exactly what I needed.

In this case the first option is free but it does not appears in the Rail Planner. I would like to choose this one but I’m not sure about its reliability. The second one for example is in the Rail Planner but the reservations are 11€ from Zürich-Milano and 10 from Milan-Firenze. Why here says valid just from Zürich-Milano?

There are some problems with ÖBB Ticketshop when you want to book two connecting trains in Italy. You can try to book only Zürich-Milano and then Milano-Firenze

Whether it's in the rail planner app or not is irrelevant. In terms of reliability, that app comes last.

@rvdborgt

Do you have information about whether you can book the night train with Interrail at an international station of NMBS? 

More specifically, do you have the B Europe product sheets for Trenitalia, which contain all the information about which trains they can book.

I hope I can book it at a station in Belgium, Ghent in particular, because I don't really trust the Italiarail website.

Should be possible:

https://public-pf.b-europe.com/nl/Producten/Details?id=0ca12a69-c9a0-4cef-9f5d-2e90a822f1dc

I haven't tried it myself though. You can also call them; delivery should then be via e-mail.

@rvdborgt 

I see that it would only be for sale in Brussels South according to the file. Would this be effective?

When I look at the sheet for Brussels France, there is a different information about the sales channel

In the meantime I have received confirmation from NMBS that they can reserve tickets for Trenitalia. Now I am still waiting for the answer whether they can also do this in Gent Sint Pieters.

Yes they can. Gent-Sint-Pieters can sell their full range of international tickets, except for interrail.

@rvdborgt 

Hmmmm, this isn't quite clear to me.

In the meantime, I have received confirmation from NMBS that they can reserve the interrail surcharges for trips with Trenitalia at Ghent station.

I wonder if NMBS can reserve me a seat at the station that I want. Because you can check which seats are available on the Trenitalia website and app.

Unfortunately, I don't see this option on the OEBB website.

I think @rvdborgt says that they can sell everything, except Interrail Passes. 

That could indeed be the case. That's why I say it for clarification.

I don't know which options they have to select a seat; that probably also depends on the country/operator etc. Possibly only type of seat, such as with table, etc. But maybe they can also indicate an exact coach and seat number. So if you note down some possibilities, they may be able to select them.

I will definitely ask if they can reserve a specific seat. 

The moment I'm at the ticket desk, I can check on the Trenitalia app which places are still available.

Hope to find a clear plan of the Trenitalia carriages, to determine which seats have a view through the windows.