For domestic trains in Spain, that is not possible, unfortunately. Spanish railways (RENFE) are basically a pain in the lower backside. They have made it quite difficult to book their trains. Booking reservations for Spanish domestic trains is currently not possible online. You can buy these reservations:

• at a Spanish ticket office
• possibly at Lisboa Santa Apolónia (they're reported to have a Spanish system there)
• at a Deutsche Bahn ticket office. You need to provide the train number.
• via Deutsche Bahn by phone (+49 30 2970). Collect at a DB ticket machine or receive via post (€5.90 per order). You need to provide the train number.
• Via a Swiss ticket office or by phone (scroll down to Switzerland).
• Via RENFE by phone, with limitations, see this page (scroll down to Spain).

The good thing is that domestic Spanish pass reservations are not quota-controlled, so you can book seats until the train is full. You can also buy all your reservations at the first Spanish station that you get to.
Train numbers can be looked up via www.raileurope.com, but not too long in advance, since RENFE don't publish their timetables a long time in advance.

I’m sorry. I should have also mentioned that that the train goes from montpellier to barcelona, so it will be a international train. Will this change anything?

Yes, that does change it. I usually insist on having exact details but assumed too much here...

You can book this train by phone via SNCF. Press #85 for English, no booking fees, reservation will be sent via email. If they claim it is not possible, just say you've booked this way before. Others have booked TGVs from France to Spain successfully this way (also recently, a few weeks ago) so it is certainly possible.

OR-as we have told here some 100+ times: go without any REServ with local trains: to Port BOu (in FR named TER), then rodalies to Bcn onward-take a little longer, but saves a whopping 30+€. The views are also much better as from hi-speed.