On the left side of the MIT APP Inventor you can find Extensions. Import the extension. You have to select the file de.ullisroboterseite.ursai2pahomqtt.aix.
Drag and drop the UrsPahoMqttClient. After that, it will appear as 'Non-visible components'
Click on UrsPahoMqttClient1 to change the Properties of the UrsPahoMqttClient1:
Protocol:
SSL
Broker:
schnupperkurs-mqtt-broker.s2.eu.hivemq.cloud
Port:
8883
TrustedCertFile
To establish an encrypted connection, the MQTT client needs the root CA server certificate of the MQTT broker. Download this certificate here: https://letsencrypt.org/certs/isrgrootx1.pem