You can use the HDMi dummy as mentioned above.

But your problem, is that you should have never plugged your monitor into your graphic card in the first place, since you don't know how to switch between the integrated GPU and External.

Leave the Graphic cards alone, and only plug your monitor to the VGA or HDMi of your Motherboard.
Try to do it, and reboot your computer (rig)
if it doesn't work, start your rig into Safe Mode.
Then reboot.
Your OS should enable your Intergrated GPU instead of your graphic card.

Also, if you connect your monitor to your VGA/HDMi of your motherboard, and go to your BiOS settings, you should be able to change to Integrated GPU.

Another way, is that if your monitor is plugged to an external graphic card, open your Graphic's control panel, and you can choose which GPU engine to be used by default. Chose your Integrated GPU. Apply, monitor could turn black, then plug it to Motherboard. Or you will need to reboot your rig while having your monitor plugged to your motherboard.