Hello ChinhN
Are you talking about the Half Fare Card for residents, or the Swiss Half Fare Card, for tourists? They are equal in terms of validity yet are destined for different customer groups.
In my opinion, it makes little sense to purchase the Half Fare Card now, as you can purchase the Saver Day Pass at half fare without actually being in the possession of the Half Fare Card. The system does not actively check whether you are eligible to purchase half fare tickets.
Your Half Fare Card must be valid on the day you want to use half fare tickets; you could purchase the half fare card even on the same day as you travel. The only requirement: it must be active before you board a public transport service.
When you purchase the (Swiss) Half Fare Card now, make sure you set the first day of validity manually to the a date in the future; if you don’t the day of the current day will be used and your Half Fare Card becomes valid instantly.
All my saying above is valid for both the Half Fare Card and the Swiss Half Fare Card.
And, to answer your question whether the Half Fare is automatically applied: no, it is not.
To consider your half fare card, you must alter the settings in your traveler data. See this post:
https://community.sbb.ch/d/26418-how-can-i-save-the-info-of-swiss-half-fare-card
Let us know if something is unclear or you need further assistance.
Kind regards,
Roland