You need to use a splitter and "Y" cable or run a separate cable.
And in case you were wondering, no you cannot use the 12VDC cable from the camera to power the IR. It is not designed that way and you will probably blow out the camera board. Maybe not immediately, maybe a few days from now or next week or next month. Just not worth as there are other ways.
The camera is designed to accept power from either the ethernet cable or the 12v cable, but not provide power from one to the other.
Now if you take a reading on the 12v, you will see some voltage (most see like 11.5v or so), but what you're reading is voltage leaking through the diode that is there to prevent drawing power from that connector and would fail under load. The PoE converter in the camera is designed to support the camera only and no external devices.
We have seen a few guys that have opened up a camera and bypassed things to get 12VDC out on that connector, but I would not suggest doing that and risk overloading and ultimately blowing the PoE board in the camera and then you are stuck with a bad camera.