1. You don't need to buy a pass before booking reservations.
2. Eurostar has limited pass holder seats but Paris-Barcelona doesn't have that anymore. That has been the case for over a year now. The number of trains is limited though so booking early is a good idea.
3. You can check availability on www.raileurope.com. reservations there are also cheaper than via Interrail since Interrail charge a €2 booking fee per person and train.

Thanks for the reply, 

2 and 3 are valuable information, but I dont understand awnser 1. I understood I would need to buy an interrail pass first and then make a reservation. Would I be able to reserve a seat without paying for a trainticket first?

Yes you can book a reservation without a pass, by using other websites than interrail.eu (which don’t have fees!).

You only need to have both at the time of boarding the train. :)

Btw there are a few ways to save money on these 2 routes, mainly by taking slower routes with a change or two. Feel free to ask for advice.

I understand now, really helpful. Thank you very much! 

Which websites allow to make a reservation for these routes without buying a pass first?

I dont know them myself.

Check here:

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

It mentions raileurope.com, b-europe.com and interrails own service. These platforms all require buying a pass first as far as I know.

Only the latter really does. The other ones are happy to accept any pass cover number such as I11111111. It won't appear on the reservation anyway.

Hmmm, yes that will probably work. I think it is better to take the following steps though just to be sure:

1. Check availability of the seat reservations you desire.

2. Buy a pass

3. Buy the seat reservations using the correct pass number.

It’s really not necessary to buy the pass first. You can if you prefer, of course, but the advice above is correct.

I think it is a dangerous advice to give to someone to enter a non-existing passnumber. It will probably be no problem in 99,5% of the cases because ticketinspectors don't get to see the passnumber when scanning the ticket or not paying attention to it. If that is no longer the case due to policy changes or a very precisely operating train inspector people could get in trouble. 

My experience travelling with interrail is some ticket inspectors only take a quick glance at my reservations while others spend two minutes checking everything. On some of the reservations I made for my upcoming travels in april the passnumber in fact is printed on the reservation document. So I would say: better safe than sorry.

In case of Eurostar, the pass number is not mentioned at all on the reservations. This is also the case for SJ.

In fact, the only case I know where they do mention the pass number is on Italian reservations booked via Trenitalia (including via Eurail/Interrail, Italiarail and probably also Rail Europe).

I currently own a Renfe reservation with a passnumber on the ticket. There are a lot of resellers for interrail reservations. All of them using scripts to generate etickets. The script may not have put the passnumber on the ticket in the past put the script can be changed next week, tomorrow or today for all we know putting the number on it.

The fact is, that technically a train inspector can block your pass in case of an incorrect number. Chances of this happening are very small, but why risk it? You will not be the first traveller by train or airline in trouble because of incorrect data and definitely not the last.

I for one am happy all details on my reservations are correct and dont need to worry the next few months up to my trip.