I am currnetly doing this, without any extra software. My strategy is to plug something into a port on the video card, while plugging my monitor into the on-board VGA port. What I got was Windows 10 extending the Desktop onto that monitor. As long as I leave it extended and don't make the monitor the "main" monitor, it works. albeit with caveats.
I had to disable extending the taskbar on both monitors, and then blindly drag the taskbar from the monitor I can't see to the one I can. I also dragged all the desktop icons over, which took some trial and error (Ctrl-A to select, and just guess where an icon is).
There is still also the problem that an app may launch on the "other monitor." To fix that, I select the app in the taskbar and then use the Windows hotkays to move it to my monitor, either Win+Shift-Left or Win-Shift-Right.
To prevent the mouse from moving from monitor to monitor, I rearranged them so that the other screen was diagonally top-left of the main one in the Display Properties (right click on Desktop or just search for it), as I don't tend to use the top right corner. If the mouse does wind up offscrreen,moving down and right very quickly will bring it back.
Finally, while I'm not sure it's absolutely required, I used the "Advanced display settings" option in the Display Settings to set my games and web browser to Run in "high performance" mode, to make sure they use the GPU and not the onboard graphics.
Yes, all this is quite tedious, but it gives me a temporary solution after my HDMI-to-VGA active adapter stopped working after a couple of years. It allows me to still use my computer until I can get a replacement.