Hello dancing queen
The Half Fare Card don’t give you a free access to the Jungfraujoch or any other railway in Switzerland. You must buy half fare tickets or you have already received separate tickets from your tour company.
If you have a Swiss Travel Pass or a GA Travelcard, you must also buy tickets for the Jungfraujoch from Grindelwald or Wengen.
You can find all informations about seats reservations and tickets here:
https://www.jungfrau.ch/en-gb/jungfraujoch-top-of-europe/
Kind regards
Danhiel