Shoutbox

It appears what you want is a ComboBoxControl, heres some example xml:

code:

---

<Control xsi:type="ComboBoxControl" Id="ComboExample">
  <Position Left="20" Top="80" Width="90" />
</Control>

---

And to add items to it with JScript:
(where WindowObject is the window returned from MsgPlus.CreateWnd())

code:

---

WindowObject.Combo_AddItem("ComboExample", "Example combo item");

---