The test we use is called the SNAP test, not sure if that is the brand name. We get those and the purple topped tubes from our vet as you need to put the blood in the tube which has an anticoagulant. I know there is also a way to do it bypassing the tube, but this is what we do.
The blood is from the jugular vein. We are lucky, Libby Rosen (Mistyrose) lives about 10 miles from us and she comes and does the blood draw while we restrain the foal. It is a two person job. She draws the blood, puts it in the tube (if not enough blood to inject into the tube you can take the top off and drop the blood in the cap, then put the blood into the mixture for the test.