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.

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.

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

@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.