Login and create a controller with proper name (Controller holds a group of different types of devices. It's like a Hub)
Click on newly created controller and add new devices. (Select device type to add different types of devices/sensors)
Copy auth key displayed on dashboard (Auth key is required to authenticate device connected via MQTT client)
Copy controller id which is useful to subscribe a topic and received messages only published by devices/sensors under that controller.
Also copy device ids which are used to identify device/sensor while communicating with server and also used to subscribe and listen to specific device/sensor messages.
Check examples for connections and basic communication using Esp32 here.