To use the Swiss Post Label Printer App you will have to add your credentials which you got from Swiss Post to the app while onboarding. If you don't have the credentials yet please visit this article.
Open the PDF which you got from Swiss Post including your Digital Commerce API credentials
Open the Swiss Post Label Printer App and start with the onboarding process.
Please add you credentials as shown here:
Generate a test label.
You can now finish the onboarding.