Content Control Check Box

You can use a 'ContentControlOnExit' macro in the document's 'ThisDocument' module for this. the following code shows how you might implement this with a checkbox titled 'MyCheckBox' and a bookmark named 'Destination' in the document.

Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
  If ContentControl.Title = "MyCheckBox" Then
    If ContentControl.Checked = True Then
      Call UpdateBookmark("Destination", "Nunc viverra imperdiet enim. Fusce est. Vivamus a tellus." & vbCr)
    Else
      Call UpdateBookmark("Destination", "")
    End If
  End If
End Sub

Sub UpdateBookmark(BmkNm As String, NewTxt As String)
Dim BmkRng As Range
With ActiveDocument
  If .Bookmarks.Exists(BmkNm) Then
    Set BmkRng = .Bookmarks(BmkNm).Range
    BmkRng.Text = NewTxt
    .Bookmarks.Add BmkNm, BmkRng
  End If
End With
Set BmkRng = Nothing
End Sub

Note that, if the checkbox is unchecked after checking, the inserted content is deleted.