[dsyrock]

This seems to work

x,y=reaper.GetMousePosition()
reaper.TrackCtl_SetToolTip("hello", x, y, false)
h=reaper.GetTooltipWindow()
time=os.time()

function countdown()
  local check=os.time()
  if check-time>=2 then  --close the tooltip after 2 seconds
    reaper.TrackCtl_SetToolTip("", x, y, false)
  else
      reaper.defer(countdown)
  end
end
countdown()

Maybe give it an empty content can somehow close it.