I want to know how to connect side signal buzzer in my honda activa.

a relay is what your turn signals etc run threw ! normally a square box with several connections stickin out of it !

Have you tried the looking at the globe in the blinker if that's fine check all the wire connections .

Contact the service department at your local showroom. They should be able to tell you the cost of the side car, any additional parts that are required, and the total cost of installation.

if you have a single dash indicator on your bike you can pick up the signal from there.
If not you can install two small buzzers or one buzzer with a diode to each of the signals, this will prevent a back feed and flashing all the lamps.

Hope this helps.