Do you want the new item added to the dropdown list, rejected, or just permitted and output to the bookmark?
As for the basic code to update the bookmark, try the following ContentControlOnExit macro:
Code:
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
With ContentControl
If .Title = "MyCtrl" Then Call UpdateBookmark("picked", .Range.Text)
End With
End Sub
Sub UpdateBookmark(StrBkMk As String, StrTxt As String)
Dim BkMkRng As Range
With ActiveDocument
If .Bookmarks.Exists(StrBkMk) Then
Set BkMkRng = .Bookmarks(StrBkMk).Range
BkMkRng.Text = StrTxt
.Bookmarks.Add StrBkMk, BkMkRng
End If
End With
Set BkMkRng = Nothing
End Sub
The code assumes your Content Control has the "MyCtrl" title.
PS: When posting code, please use the code tags. They're on the 'Go Advanced' tab.