Whaikainnauss93
You will “use” the half fare card only in the train/bus etc. in case of a ticket control: It will then be also checked as proof that you are entitled to travel with a 50% discounted ticket instead of a full fare ticket.
Whether you (already) have a valid half fare card during the online ticket purchase is irrelevant, as this won’t be cross-checked (same if buying tickets from a vending machine). Simply make sure that you purchase the tickets at ½ price by selecting the correct travelcard in the passenger details (as shown in Roland’s screenshot), so the system calculates the correct ticket price.