There are overloads of ActionLink which take a fragment parameter. Passing “section12” as your fragment will get you the behavior you’re after.
For example, calling LinkExtensions.ActionLink Method (HtmlHelper, String, String, String, String, String, String, Object, Object):
<%= Html.ActionLink("Link Text", "Action", "Controller", null, null, "section12-the-anchor", new { categoryid = "blah"}, null) %>