How to force HDMI output on NVIDIA when tit “thinks” there isn’t anything connected?

I have one of those hdmi displays for a raspberry pi, they provide no display information(I think it’s called hetd info). The way it works in the raspberry pi is that you have to force it to enable HDMI in the config file on the sd card. Now I want to connect it to my Ubuntu PC, which has a NVIDIA card and has proprierty NVIDIA drivers. Since it’s not detected(as expected), it outputs nothing, so I’m looking for a way to force it to output a 800*480 image over HDMI, even if it thinks nothing is connected.

How would I do that?

Thanks!