You do not need to add the HFC to the app for it to display discounted prices. If you want to buy a discounted ticket just indicate where you add the passengers that they have the Half Fare discount. There is no need to link the card. You will always be asked to show both your ticket and the HFC when the ticket inspector comes along.
If you create an account first before buying the HFC it gets added to the app automatically. If neglected to do that there is no way to add it afterwards.