The horn gets power by earthing the contact ring on the steering column. On the back of the boss, there should be a spring-loaded contact pin that touches the brass/copper ring on the steering column. Inside the boss a wire should lead from the other end of the spring-loaded pin to the horn button. The other contact on the horn button (if it has one) needs to be earthed to one of the screws on the boss, or sometimes there’s a ring provided with the boss kit or steering wheel that has a spade terminal on it, and that ring gets sandwiched between the steering wheel and the boss. Some horn buttons don’t have a separate earth wire, and instead have a sort of spring clip that must touch either the boss or some other earth point. If it’s not clear, maybe send a couple pics of your boss and horn button/steering wheel so we can determine what type set-up it has.