When adding the publish method to main.qml in example: https://doc.qt.io/QtMQTT/qtmqtt-quicksubscription-example.html, a -1 (error) is returned.

 
MqttClient {
        id: mqttclient
        port: 1883
        hostname: "localhost"
    }
 
    Button {
        id: connect
        text: qsTr("Connect")
        onClicked: {
            console.log(mqttclient.connectToHost())
        }
    }

    Button {
        id: publish
         text: qsTr("publish")
        onClicked: {
            console.log(mqttclient.publish("mytopic","mytestmessage"))
        }
    }
 
As suggested by Maurice Kalinowsky, this is mostly caused by a failing auto-conversion from QString to QMqttTopicName.