Might work, seems to work, donno, try it

RegisterNetEvent("code")
AddEventHandler("code", function()
    Citizen.CreateThread(function()
        if IsPedInVehicle(GetPlayerPed(-1), vehicle, false) == false then
            print("You Are Not In A Vehicle")
            return;
        end

        local playerVehicle = GetVehiclePedIsIn(GetPlayerPed(-1), false)
        if IsVehicleSirenOn(playerVehicle) == true then
            print("Off")
            SetVehicleSiren(playerVehicle, toggle)
        else
            print("On")
            SetVehicleSiren(playerVehicle, toggle)
        end
    end)
end)