TMessageBox.Default
From OpenEUO
You can set TMessageBox.Default to the value of the button you'd like selected as default
0 is always the left-most button and the selection moves right as the number increases.
The example below shows cancel selected as default.
form = Obj.Create("TMessageBox")
form.Button = 3
form.Default = 2
form.Title = "Brian G's Tree Chopper"
form.Show("Path file not found!\nWould you like to make a new one right now?")