I'd STRONGLY recommend putting a relay between the horn and the battery. It's a super simple operation after all - wire the stock cable to the relay, and the relay to the horn and the battery.
These air horns can suck down 20 amps, and the stock wiring is almost certainly not rated to do that, the little beeper that's there usually has to use almost no current at all. It will honk just fine until it eats the fuse or something else not desirable.
At least that would be my opinion. A relay costs next to nothing, the work is super easy, and it's just so much better to be safe than sorry. This bike already has issues with too much current going through the keyswitch, too.
Edit: also, every time you push your horn button now, 20 amps will flow through that so that will probably also fry sooner or later without a relay.
Get a relay.