First, login to your Supertext AG account with your normal username and password:
https://www.supertext.ch/de/login
Then click on Einstellungen below Konto.
Scroll all the way down until you see the part “API Authentifizierungs Token”
Here you see your token. Use this and your username in the system that sends requests to the Supertext API.
If you don’t have an account yet, you can create one here:
https://www.supertext.ch/de/signup