client side
RegisterNetEvent('fivem-seatbelt:enable')
AddEventHandler('fivem-seatbelt:enable', function()
 if IsPedInAnyVehicle(GetPlayerPed(-1), false) then
 beltOn = not beltOn
     if beltOn then
        TriggerEvent("ls:notify", "on.", 0.080)
        TriggerEvent("pNotify:SendNotification",{
                text = "Bälte: PÅ",
                type = "success",
                timeout = (1000),
                layout = "bottomCenter",
                queue = "global"
        })
     else
        TriggerEvent("ls:notify", "off.", 0.080)
        TriggerEvent("pNotify:SendNotification",{
                text = "Bälte: AV",
                type = "error",
                timeout = (1000),
                layout = "bottomCenter",
                queue = "global"
         })
    end
  else
    TriggerEvent("ls:notify", "Du är inte i en bil.", 0.600)
    TriggerEvent("pNotify:SendNotification",{
            text = "Du är inte i en bil.",
            type = "error",
            timeout = (1400),
            layout = "bottomCenter",
            queue = "global"
        })
  end
end)

config

Cfg             = {}
Cfg.DiffTrigger = 0.255
Cfg.MinSpeed    = 19.25 --THIS IS IN m/s
Cfg.Strings     = { belt_on = 'Säkerhetsbälte ^5 på^0.', belt_off = 'Säkerhetsbälte ^1 av^0.' }