Including an anchor tag in an ASP.NET MVC Html.ActionLink
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) %>
I would probably build the link manually, like this:
<a href="<%=Url.Action("Subcategory", "Category", new { categoryID = parent.ID }) %>#section12">link text</a>