How can I set up pre fill for city/town based on post code? (German/Austria/Switzerland)

One built-in way you might approach this is to use a Drill Down question and its subsequent set up. This would require that you populate three columns with "Post Code", "City", "Country". You would then upload this into Qualtrics, as described in the instructions found here. This way only cities/towns and countries that apply to that post code are shown, while still allowing the respondent to select the appropriate (if needed). I suspect in most cases the post code will only show one city/town and one country, thus limiting and "auto-populating" the appropriate content to those fields.