some Razor controls

drop down list:

@Html.DropDownListFor(m => m.SelectedCategoryId, new SelectList(Model.AllCategories, "Id", "Name"))

 
The category value will end up the value of the hidden variable in this case:

@ViewBag.Category

@Html.Hidden("Category")

 
Otherwise here is a name first, value second way of writing a hidden variable:

@Html.Hidden("Foo","Bar")