[Platine]

OK, thanks everybody, addon works correctly.

ST_Tooltip=CreateFrame("GameTooltip", "ST_Tooltip", UIParent, "GameTooltipTemplate"); 
ST_Tooltip:SetOwner(UIParent, "ANCHOR_CURSOR");
 
GameTooltip:HookScript("OnTooltipSetItem", function(self) 
  ST_Tooltip:ClearLines();
  ST_Tooltip:AddLine("My frame", 1, 1, 1);
  ST_Tooltip:AddLine("Second line", 1, 1, 1);
  ST_Tooltip:Show(); 
);