What you need is
Code:
Sub Macro1()
Dim oCC As ContentControl
Set oCC = ActiveDocument.ContentControls.Add(wdContentControlDropdownList, Selection.Range)
With oCC
.Title = "My List Box"
.Tag = .Title
.LockContentControl = False
.SetPlaceholderText , , "Select Fruit"
.DropdownListEntries.Add Text:=.PlaceholderText, value:=""
.DropdownListEntries.Add Text:="Apples", value:="Apples"
.DropdownListEntries.Add Text:="Pears", value:="Pears"
.DropdownListEntries.Add Text:="Plums", value:="Plums"
.DropdownListEntries.Add Text:="Grapes", value:="Grapes"
End With
Set oCC = Nothing
End Sub
or take a look at
Insert Content Control Add-In