diff --git a/autotests/kstartupinfo_unittest.cpp b/autotests/kstartupinfo_unittest.cpp
--- a/autotests/kstartupinfo_unittest.cpp
+++ b/autotests/kstartupinfo_unittest.cpp
@@ -1,6 +1,6 @@
 /* This file is part of the KDE libraries
 
-    Copyright 2012 David Faure <faure@kde.org>
+    Copyright 2012,2019 David Faure <faure@kde.org>
     Copyright 2012 Kai Dombrowe <just89@gmx.de>
 
     This library is free software; you can redistribute it and/or
@@ -22,6 +22,7 @@
 #include <QSignalSpy>
 #include <qtest_widgets.h>
 #include <QX11Info>
+#include <QWidget>
 
 #include <xcb/xcb.h>
 
@@ -62,6 +63,7 @@
     void checkStartupTest();
     void createNewStartupIdTest();
     void createNewStartupIdForTimestampTest();
+    void setNewStartupIdTest();
 
 private:
     KStartupInfo m_listener;
@@ -305,6 +307,23 @@
     QCOMPARE(id.mid(index + 4).toULongLong(), 5u);
 }
 
+void KStartupInfo_UnitTest::setNewStartupIdTest()
+{
+    {
+        QWindow window;
+        const QByteArray str = "somefancyidwhichisrandom_kstartupinfo_unittest_2";
+        KStartupInfo::setNewStartupId(&window, str);
+        QCOMPARE(KStartupInfo::startupId(), str);
+    }
+
+    {
+        QWidget widget;
+        const QByteArray str = "somefancyidwhichisrandom_kstartupinfo_unittest_3";
+        KStartupInfo::setNewStartupId(&widget, str); // deprecated
+        QCOMPARE(KStartupInfo::startupId(), str);
+    }
+}
+
 QTEST_MAIN(KStartupInfo_UnitTest)
 
 #include "kstartupinfo_unittest.moc"
diff --git a/src/kstartupinfo.h b/src/kstartupinfo.h
--- a/src/kstartupinfo.h
+++ b/src/kstartupinfo.h
@@ -30,7 +30,8 @@
 #include <QObject>
 #include <QString>
 #include <QChildEvent>
-#include <QWidgetList>
+#include <QWidgetList> // for WId
+#include <QWindow>
 
 #include <sys/types.h>
 
@@ -109,7 +110,20 @@
      * or a new one, before being shown. Note that this function is usually
      * needed only when a window is reused.
      */
-    static void setNewStartupId(QWidget *window, const QByteArray &startup_id);
+    static void setNewStartupId(QWindow *window, const QByteArray &startup_id);
+
+    /**
+     * Use this function if the application got a request with startup
+     * notification from outside (for example, when KUniqueApplication::newInstance()
+     * is called, or e.g. when khelpcenter opens new URL in its window).
+     * The window can be either an already existing and visible window,
+     * or a new one, before being shown. Note that this function is usually
+     * needed only when a window is reused.
+     * @deprecated since 5.62, use setNewStartupId(QWindow *) instead
+     */
+#ifndef KWINDOWSYSTEM_NO_DEPRECATED
+    KWINDOWSYSTEM_DEPRECATED static void setNewStartupId(QWidget *window, const QByteArray &startup_id);
+#endif
 
     /**
      * If your application shows temporarily some window during its startup,
diff --git a/src/kstartupinfo.cpp b/src/kstartupinfo.cpp
--- a/src/kstartupinfo.cpp
+++ b/src/kstartupinfo.cpp
@@ -777,9 +777,18 @@
 
 void KStartupInfo::setNewStartupId(QWidget *window, const QByteArray &startup_id)
 {
-    bool activate = true;
+    // Set the WA_NativeWindow attribute to force the creation of the QWindow.
+    // Without this QWidget::windowHandle() returns 0.
+    window->setAttribute(Qt::WA_NativeWindow, true);
+    setNewStartupId(window->window()->windowHandle(), startup_id);
+}
+
+void KStartupInfo::setNewStartupId(QWindow *window, const QByteArray &startup_id)
+{
+    Q_ASSERT(window);
     setStartupId(startup_id);
 #if KWINDOWSYSTEM_HAVE_X11
+    bool activate = true;
     if (window != nullptr && QX11Info::isPlatformX11()) {
         if (!startup_id.isEmpty() && startup_id != "0") {
             NETRootInfo i(QX11Info::connection(), NET::Supported);
@@ -798,7 +807,6 @@
         }
     }
 #else
-    Q_UNUSED(activate)
     Q_UNUSED(window)
 #endif
 }