// customitem.h
#pragma once

#include <QObject>
#include <QQuickItem>
#include <QtQml>

class CustomItem : public QQuickItem
{
    Q_OBJECT
    QML_ELEMENT
public:
    CustomItem();

signals:
};

// Main.qml
import QtQuick

import com.mycompany.qmlcomponents

Window {
    Item {}
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    CustomItem {}
}

qt_add_qml_module(appjust_qml
    URI com.mycompany.qmlcomponents
    VERSION 1.0
    QML_FILES
        Main.qml
        SOURCES customitem.h customitem.cpp
)

QQuickItem was not found. Did you add all imports and dependencies? [import]

Type CustomItem is used but it is not resolved [unresolved-type]