If that field is working then you must have a bookmark in the document of the same name
Code:
Sub ResetBookmark()
Dim sBkmk As String, aRng As Range
sBkmk = "MyName"
If ActiveDocument.Bookmarks.Exists(sBkmk) Then
Set aRng = ActiveDocument.Bookmarks(sBkmk).Range
aRng.Text = "Hello from the other side"
ActiveDocument.Bookmarks.Add Name:=sBkmk, Range:=aRng
End If
End Sub