It turns out I had to use GameTooltip:AddDoubleLine() instead to create two columns. Also, in TooltipTextureInfo I changed "region" to "RightLine" to make an icon appeared at the right column. Now everything seems to work. This is the code:
Lua Code:
-- Title
tooltip:AddDoubleLine(label, " ", 1, 1, 1, 1, 1, 1)
-- Texture
local tooltipTextureInfo = {
anchor = Enum.TooltipTextureAnchor.RightTop,
margin = { left = 0, right = 0, top = 0, bottom = 0 },
region = Enum.TooltipTextureRelativeRegion.RightLine,
}
local pathto = iconfolder .. iconfile
tooltip:AddTexture(pathto, tooltipTextureInfo)