Simply pass the id of the show with the show parameter
Show url: https://www.ardaudiothek.de/sendung/kalk-und-welk/10777871/
Id: 10777871
Feed url: http://localhost:8080/index.php?show=10777871
If you only want to receive the n newest episodes, pass the latest parameter too:
Feed url with 10 newest episodes: http://localhost:8080/index.php?show=10777871&latest=10