No, you don't connect anything to your onboard video. You pretend that there is something connected, while in fact there is nothing. In the linked guide, you do this by using "another display not detected" and using the entry "try to connect anyway..." that is shown in step 6 of the guide.
If you do this, you have a display without connected monitor. The only problem with this is, that there may actually be applications that open windows on that - they are invisible. You are also able to move the mouse there, so it vanishes from your view.