Modern Vespa

I need a permanent +wire.
I bought a complete system for when i forget my turn signals.
This system should remind me with beeps after the turn signals has blink 20 times.
Can i use a +wire from under the dashboard without problems with the canbus system of my MP3 250 ie LT 2009 or do i have to lay a wire (with fuse) directly from the battery?

Run a wire directly to the battery, unless you have a Acc (a.k.a. switched +12V) wire from a installed accessory such as the Piaggio heated grips, heater blanket, Piaggio/TomTom GPS mount, Piaggio bluetooth speaker, Piaggio PMP, etc, etc, etc.

Never splice directly into a vehicle with a canbus system.

I tought so.
One time a friend of mine had a canbus problem (has the same MP3 as me) when he installed a extra brake light.
He spliced the wire for the normal brake lights.
He took the brake light if and the canbus was normal again.

Best option is to install a isolator fuse block.

That way, you have a real switched +12V source, you have separate protected circuits, and it is designed to play nice with the canbus.

https://www.amazon.com/Show-Chrome-Accessories-13-311-Electronic/dp/B01M4K65UO

most time it isn't a canbus problem but a amp problem

a 250 does have about 2 amps for aux so heated grips navigation and a third brake light could be to much

It never a good idea to use a wire directly from the battery without switching it with the ignition switch

Direct from the battery even with a fuse will be a parasitic draw, how much depends on the device

Does the signal wire for the device provide the power for the buzzer or is there a separate + wire?

Use a keyed + wire to drive the coil of a relay
Run a fused wire from the battery through the contacts of the relay, for the buzzer device or other accessory

Myself I would use the headlight wire to power the relay coil, it cuts off when the starter spins