This example adds a combo box control to the command bar named "Custom" and fills the list with two items. The example also sets the number of line items, the width of the combo box, and an empty default for the combo box.
Set myControl = CommandBars("Custom").Controls _
.Add(Type:=msoControlComboBox, Before:=1)
With myControl
.AddItem Text:="First Item", Index:=1
.AddItem Text:="Second Item", Index:=2
.DropDownLines = 3
.DropDownWidth = 75
.ListHeaderCount = 0
End With