Its impossible. RENFE=the state railway for ESpana has very, very limited options to book online (think 0). Also note that this long route is by far and with *** the most cumbersome line, due to high demand. It is possible (but they will never tell you this) to go via Madrid and use the normal hourly AVE=hispeed trenes, but this will lengthen triptime and cost (you pay per train), but also give many more options.

If you look at the post  below there is a detailed answer describing how reservations for domestic Spanish trains can be made.

Thank you for the info. Will wait till we arrive in Spain