How do I attach the cord from a pendant light to a nipple/threaded shaft?
Unlike some lamps this doesn't hang from a metal tube with the cord running through it.  It hangs from the cord.  Shove it through the nipple and tie a knot above it.
There are S-shaped plastic dongles you can attach to the cable above the nipple, but those cost about 10 cents more than a knot and do the same thing.  Hopefully your ceiling box already has a canopy (decorative cover) as this lamp probably doesn't come with one right?
If you want to spend a few cents there are plastic bushings for these nipples you could stick into the top to protect the cable, and there are compression fittings where you shove a plastic bushing in the nipple and then a plastic collar screws down on it.  IMO none of those is necessary unless you plan to swing from the lamp.
EDIT: I'm not sure why @threephaseEel asked for a picture of your ceiling box, but it's a good thing.  Now that I've seen it, I think you should ignore this answer.  You cannot use a standard crossbar and nipple with that box because the mounting hub you have for a chandelier will obstruct it.
You either need to

perform surgery on the box, removing that hub, which I don't recommend with that delicate K&amp;T wiring OR
Buy an appropriate canopy kit, which I'll recommend in a new answer.