Once you have the app on your phone if you go to the menu (three horizontal lines in the upper right), click on your name you will get to a menu for putting in all your billing and credit card information. I have set mine up, but have never used it. Rather pay cash or present a card as I am just an old fart and am more comfortable that way.
BUT, while the Taxi is great for getting from the train station or airport, you do realize it is not how you get around town except in special circumstances. For getting around town you need good shoes and YOU NEED a pass for the public transportation.
For that matter, if you are not carrying much luggage and depending on where your accommodations are, the public transportation may be a good way from the train station to your accommodations (which station and where you are going and the time of day, might change my opinion).
You might as well hit the ground sightseeing and the public tranportation will get you a eye opening introduction to the city. All the train station discussed are in the part of the city that was constructed prior to WWII. The entirety of the pre-WWII city is neo-classical/barouque/gothic with some Art Nouveau / Art Deco / Bauhaus thrown in for good measure and the trams from the train stations to the part of town you should be staying in run through it. Here is another individual's opinion on the looks of the city: https://youtu.be/0nd9DuDGCz0?si=BsIK1a8oAKjm77ef