A basic example of using PubSubClient with WifiNina library to access a local broker. This sketch just connects to WiFi and then subscribes to the topic "test/hello" using a username & password to access the broker. Any message recevied is then printed to the Serial for debugging.
Note this example is based on using PlatformIO with the PyPortal M4 Environment although it should work with any device that supports the WiFiNina library.
This file is used to define various passwords and configuration environment specific, the following values need to be designed or removed from the code to compile
#define WIFI_NAME ""
#define WIFI_PWD ""
#define MQTT_USERNAME ""
#define MQTT_PASSWORD ""
#define MQTT_BROKER_IP3 0
#define MQTT_BROKER_IP4 0
Note that as this is a local broker is it assumed it will be on address 192.168.X.X so just defined last two numbers.
To access a external broker such as a cloud broker you need to change to use MQTT client creation to use a string instead of a IP address. The use of a hostname for a local broker requires mDNS which would add lot of extra code to this example.