Hello Ghimbapuek83
It makes little sense to purchase mountain excursion tickets in advance. The weather in Switzerland is not predictable, not more than two days out. Would you also go with bad weather, rain and clouds? Mt. Titlis his one of the most expensive mountain excursions in our country, I am not sure if you are willing to spend a fortune to finally end up in the clouds. Same for Jungfraujoch…
If you decide you want to visit Mt. Titlis, I suggest purchasing the tickets at the SBB travel centre in Lucerne or any other SBB travel centre the day before only.
To answer your question: you do not need a customer number to purchase hald fare tickets. Simply download the SBB Mobile app and follow the instructions I have given to another user in this post; they also wanted to know how to activate the half fare option for Swiss Half Fare Cards:
https://community.sbb.ch/d/26418-how-can-i-save-the-info-of-swiss-half-fare-card
Please let us know if you have got more questions.
Kind regards,
Roland