Can not be done online anyway.

Usually 3 or 4 month advance, may even differ who runs train you want (can be FR-SNCF or ES-RENFE/AVE).

What most people here do not really understand is that REServing just a seat is not something that needs to be done asap-that is more for the few cheapest tickets-prices for these rise when lowest quota are done.

IF this is the only trip you do that day, it is very easy, but more time-consuming, to do the old-fashioned way via rodalies to Cerbëre, then local TER onward with out any surcharge. The sights are also much better that way