[Harven]

I don't know about the WoW api similarity but I can help you by showing you how i would do it:

First to define some button in you XML (inside <Controls> element, inside <TopLevelControl>):

<Button name="$(parent)OpenDropdown" inherits="ZO_DropdownButton">
   <Anchor .../>
   <OnMouseDown>
      ToggleMyDropdown(self)
   </OnMouseDown>
</Button>

Then in your lua code:

local hidden = true
function ToggleMyDropdown(control)
   hidden = not(hidden)
   if hidden then
      ClearMenu()
   else
      ClearMenu()
      AddMenuItem("Item1", function() d("item1") end)
      AddMenuItem("Item2", function() d("item2") end)
      AddMenuItem("Item3", function() d("item3") end)
      SetMenuHiddenCallback( function() hidden = true end )
      ShowMenu(cotrol,1)
   end
end

This is not working example so you should figure out where to put the button, what attributes to set and so on.

Edit:
the GUI section is here already for some time now

Edit2:
added SetMenuHiddenCallback()