API:

Add custom item to the item list

(void) AddCustomItem(Plugin plugin, int itemId, ulong skinId)
(void) AddCustomItem(Plugin plugin, int itemId, ulong skinId, string customName)
Example:

Its recommended to delay the call a little bit to make sure Loottable is loaded

private void Init()
{
  timer.In(1f, () =>
  {
    Loottable?.Call("AddCustomItem", this, -946369541, 2664651800, "High Quality Fuel");
  });
}