Single select drop-down lists

I think what you mean is called a ComboBox in MaxGui.

From the help:

' createcombobox.bmx

Strict 

Local window:TGadget
Local combobox:TGadget

window=CreateWindow("My Window",30,20,200,200)

combobox=CreateComboBox(4,4,120,22,window)
AddGadgetItem combobox,"Short"
AddGadgetItem combobox,"Medium"
AddGadgetItem combobox,"Fat",True
AddGadgetItem combobox,"Humungous"

While WaitEvent()
	Select EventID()
		Case EVENT_GADGETACTION
			Print "eventdata="+EventData()
		Case EVENT_WINDOWCLOSE
			End
	End Select
Wend