Modern Vespa

I've recently been given a horn.
I have a switch, spare battery and wondering how to go about it. Not looking to add to existing wiring. Wanting to keep it independent, thanks.

You should use a fuse too.

Is this a decorative novelty horn or just a second internal horn?

Are you looking to amplify the horn by adding a second? If so it might make more sense just to replace the OEM with a higher decibel aftermarket air horn.

It sounds like this is a purely standalone situation with a button, battery, and horn.

I'm leaving existing horn as is.
The new one has a special effect sound which I like. So it's really a novelty sounding horn. I would like to keep it independant

Wire directly from the positive terminal of the battery, to a fuse, to the switch, to the horn, then back to the negative terminal of the battery.

I would use a momentary contact pushbutton switch.

How much power does the horn pull? Some horns pull a lot of juice and will overload a momentary switch. You might need to run a larger gauge wire to a relay then connect the horn to the relay. Use a momentary switch to activate the relay. Wiring horns isn't always simple.

Ice cream truck or General Lee?

Ba-na-nah-nah-nah-na-nah-nah-nah-nah-nah!

(It's all there)

Ooh, that adds so much wind resistance!