See below the configuration tab in the 🔧 wrench section to find the setup URL.
In case of problems with the configuration panel you can find the setup URL in flohmarkts logfile in /var/log/__APP__/app.log if you look for entries like this. To get the entry into the logfile you should at least open your flohmarkt web page once at [https://__DOMAIN__/](https://__DOMAIN__/).