@Annie.  It can be wise to add that a pre-reserved seat reservation has to be collected in Spain within 72 hours of the phone call.

Hello!  In general you need to be logged in and have selected your created trip and travellers to use the reservations self service system. 

https://www.eurail.com/en/book-reservations

However, please let me explain that unfortunately, reservations for domestic trains in Spain cannot be made online. The only way to make reservations for domestic and long-distance trains is directly at the ticket offices of local Long Distance stations (not commuter stations). Find a list of long-distance stations here:

https://www.eurail.com/content/dam/pdfs/Reservations%20in%20Spain%20-%20stations.pdf

You can make reservations on the day of travel or up to 3 months in advance. It is also possible to pre-reserve a seat by phone, by calling +34 91 232 03 20 (Renfe phone sales). Please make sure to check this page for more details:

https://www.eurail.com/en/get-inspired/top-destinations/spain-train

TIp: once in ESpana-plan ahead and make all RES-about any long-dist train needs to, and I guess you will not stay in Slm forever, right then&there for all. Expect a few ´completo´ (not very likely for this treno) too, so have plan B for just in case.

There isn’t a way to book this online, reservations for Spanish trains can only be made at Spanish stations and possibly a few other European countries.

Read this post for more details.