Set value for textbox in action

I have a Textbox rendered by an HtmlHelper

 <%= Html.TextBox("CategoryTitle",Model.CategoryTitle) %>

I post to an action. In the action I manually change the value for CategoryTitle and need to display this new value to the user, but the original value from the post is taken.

 public ActionResult Textboxer(CategoryViewModel model)
 {

     model.CategoryTitle = model.CategoryTitle + "val1" ;


     return View("Textboxer", model);
 }

I need to keep the default behaviour of the Textbox (getting red when invalid).I dont feel like writing my ow开发者_开发技巧n helper for it.

Is there an easier way?

ModelState["CategoryTitle"].Value = 
    new ValueProviderResult(NeededValue, NeededValue, CultureInfo.CurrentCulture);

The reason behind this, I believe, is that Html.TextBox re-use "saved" values from ModelState whenever possible.