Bro if your bike has neutral indicator then you can wire the bell in parallel to neutral indicator take bulb.
The neural gear indicating has one side connected to positive of battery one side is grounded via the gear shift drum on engine.
Some bikes have proper gear indicator which shows all gears. That has more contacts and a decoder with those.
Do you have the bell? What voltage it requires?
Share more details. And why you want buzz on neutral light?
You can get tweeter from any pc motherboard.