Hi, you can check on renfe.com if seats are left on this trains, trying to book a normal ticket. Pass holder can book a reservation for all available seats in the class they have in the pass or lower. But RENFE in spain has confusing names for classes. 

The 2 first have trains-the superfast AVE-ev hour, so on slack times there will be a seat. Quell all those fears.

To go to Msl=FR, =over border, this will cost an awful lot more, there are probably quota for passholders-so for that it is wise to inform yourself about ALTernatives-and there are many besides this 1 and only direct train. Can do it even without any extra pay on local=regional trains. Can set app to that-just play a bit more and get informed. This meansa slower ride, but also much more to see= what trains were intended to be so good for.

Also look at a map: you do not need to go far north as Valence to reach Msl-change Tarascon or Avignon (unless of course you have some reason to go there)

Hi! Thanks for your response!! 
I’m still confused a bit- how do I book the ticket at the rate of the reservation add on and not paying for the whole ticket. I can’t find an option on Renfe to do this. Or do I have to purchase a whole ticket aside from the pass I bought through Eurail? 
 

Hi! 
i noticed that with the valence and I’m honestly unsure why it had taken me that way when I plugged it in. 
I’m very lost on how to go about doing it without extra pay? I’m bummed because I bought the 15 day pass and ended up having to purchase a deprecate train from london to Paris so trying to avoid this. I’ve been attempting it for quite some time and think my brain has reached its limit on finding a solution! 
Any more help is appreciated :) 

Here are the known ways of buying reservations on domestic Spanish trains. 

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105/index2.html?postid=18419#post18419