Most people either attach it to the same loops where the chains get attached, or on the end of the clevis pin (the big pin that holds the hitch in the receiver), just behind the cotter pin (the small pin that holds the clevis pin in place). However, there have been accidents where the entire receiver assembly drops off the tow vehicle either due to rust or the bolts pull out... and when the brake cable is attached to it... no emergency brakes.
It is best to find something you can attach it to directly on the car. Some foreign cars have tie-down loops underneath that are used during shipping which are perfect, or you might find two closely-spaced holes underneath.