Usually to the neg side of coil. But if it is a autometer gauge they tell you not to hook them to MSD coils, don't know why but they do, I've yet to figure out how to hook one to a msd coil.
If it is a msd electronic ignition (capacitive discharge or multiple spark) you need to consult the msd web site. I think you need some sort of adapter.
If it is just a coil i would treat it as a coil. Try it and see. the tach will either work right or not.