Mine, in cars that never had one had a grey circle piece of plastic covering it. You just pull out the grey (or whatever colour the carpet is in your car usually) circle & there will be a hole there to screw in the anchor. Most car seats will come with an anchor.
It could be on the floor behind the seat in a wagon, on the boot wall right at the back under the door in a hatch, or on the parcel shelf in a sedan