There is no way for Spanish reservations for passholders to be made online, they can only be done at Spanish station ticket offices.

You can call the Spanish operator RENFE and pre-reserve tickets to be collected and paid for at one of their ticket offices but this can only be done at most 72 hours before you collect or they expire.

Details are given under the Spain tab here https://www.interrail.eu/en/plan-your-trip/reservations/how-do-i-book-my-reservations

As normal ticket holders can buy up the seats there is no guarantee there will be spaces available so the earlier you can get them the better.

It is possible if you are in Germany before Spain to get them from German rail ticket offices there as well, detailed in this thread

That’s wild . So much has to depend on the train having an available seat then, that’s sad. At night too. 
thank you. I’ll be in Portugal a few days before, you think I can book through them?

Maybe - we haven't had any reports yet, positive or negative, from people trying to book a Spanish domestic reservation in Portugal. So do let us know your experience if you try there.

There is a lot of scaremongering about Spanish reservations, especially on these popular routes. Since the advent of low cost carriers (outside the Interrail/Eurail network) many Spaniards are using those carriers leaving plenty of space on most RENFE services. 

RENFE operate many services between MAD and BCN (almost half hourly) so getting a seat should not be a big problem - just a chore queueing.

i got scaremongered , even though its a monday evening train, I got worried of getting stuck without a seat, but after thinking about it, its very unlikely there will be high traffic to barcelona at monday night, on several different trains from madrid lol. I felt its unrealistic, but without experience i didnt know. Thank you for your input, I will still try to get a train if i can because I want to see the views while travelling on a train through spain. Are the views nice/worth it?

Personally I never worry about the views on high speed trains - you go past them so fast there is always something new - and then it is gone. Enjoy the onboard experience, especially with a glass of something nice.

Question. I just got my reservation today mad to bcn from the cue at the station. The reservation is at 15:30, but on Eurail pass there is only 15:00 and 16:00. The lady at the counter said they have to match, but I have no options for them to match. Will I have an issue on the train?

She is correct, you need to add the correct train you are taking.

If a train you want to travel on doesn’t show in the planner then scroll down and use the add manually option, use the details from your reservation and it will show on your Pass ticket under the QR code.

Great, I fixed it. Thank you for the prompt reply!!

The train at 15:30 from Madrid Atocha is in the app but since it is overtaken by the 16:00 departure, the planner won't show it with the default options. If you e.g. add via Zaragoza, then you will see it.