var bookmark = {
   id: 'id',
   description: 'description',
   notes: 'notes'
}

this.bookmark = ko.observable();

this.bookmark.subscribe = function(bookmarkWithNewNotes) {
   //use the bookmarkWithNewNotes.id to update the bookmark in the db
}

this.bookmark(ko.mapping.fromJS(existingBookmark));

<div databind="with: $root.bookmark" >
    Notes
    <textarea class="userNotes" rows="10" data-bind="value: notes" ></textarea>
  </div>