It depends on what family of motherboard you have.
If you have a a P series (which disables the onboard graphics totally) plug it into the discrete graphics card.
If you have a Z68, H 61 or H68) series, its a bit more complicated - you need to install a piece of software called virtu which lets you use both at once, and switch off the discrete graphics as needed.There's a small performance drop when this is done, apparently. You should be able to get virtu off your motherboard's driver website.
If you're using virtu, you'll need to plug your monitor into your onboard video card.I'd suggest confirming this with your motherboard and software documentation however, since i've gone off reviews in writing this answer - i wasn't aware that virtu worked on H series motherboards.
There's a little more information on virtu here which confirms were you plug in the display out on the motherboard, and it goes into some detail on the install process, and has some benchmarks
Well its 2024. Virtu's dead but you can run discrete and integrated GPUs together with the correct bios settings on most modern PCs. I'm currently running one of my displays on the integrated graphics, and the rest on the discrete GPU.
Windows 10 and 11 will render applications on whatever GPU the monitor is on unless set explicitly
o.O
:-|