I believe you will need something like (sorry only working with LUA not xml)
Lua Code:
local line = CreateControl(nil, <youraddonwindowobject>, CT_LINE)
line:SetAnchor(TOPLEFT, <youraddonwindowobject>, TOPLEFT, 0, 10)
line:SetAnchor(TOPRIGHT, <youraddonwindowobject>, TOPRIGHT, 0, 10)
to see methods of line
http://wiki.esoui.com/Controls#LineControl