You can book tickets with half fare reduction even without having the half fare card. That you have the card (and that it is active) is not checked during purchase. it is during travel that this is checked. On the train you will be asked to show both the ticket and the half fare card.
Note that it is not usually necessary to buy tickets far in advance. Normal tickets are always the same price. Only super saver tickets are cheaper in advance, but these are not flexible.