If created an account on www.sbb.ch and bought the half fare card under that account it will automatically appear in the app. If you did not do that there is no way to load the pass in the app.
But you do not need to have the HFC in the app to buy discounted tickets. In fact, you do not even need that card at all to buy discounted tickets.
The HFC allows you to travel with half priced tickets, it is not needed to buy half priced tickets. When buying tickets just indicate that you will have a half price card under reductions, and the prices will be reduced.
Know however that there is normally no need to buy public transport tickets in advance. Most people just buy them before boarding.