Here is a complete listing of the example source file.
import QtQuick 2.11
import QtQuick.Controls 2.1
import QtQuick.Layouts 1.3
 
 
ElementSettings {
    element: instrument
 
    property BasicDiscreteInstrument instrument
 
 
    GroupBox {
        title: qsTr("Custom properties")
 
        Layout.fillWidth: true
 
        GridLayout {
            columns: 2
            Label {
                text: qsTr("Symbol:")
            }
 
            TextField {
                text: instrument.symbol
 
                onTextChanged: instrument.symbol = text
            }
        }
    }
}