Basic Search Not Working

I know I have some silly error in here.  But I cant see it.

I am adapting the search from a tutorial to work for me...but it isn't.

Controller:

 public async Task<IActionResult> Index(string searchString)
        {
            ViewData["CurrentFilter"] = searchString;

            var site = from s in _context.Sites
                       select s;

            if (!String.IsNullOrEmpty(searchString))
            {
                site = site.Where(s => s.SiteName.Contains(searchString));
            }
            return View(await _context.Sites.ToListAsync());
        }

View

<form asp-action="Index" method="get"><div class="form-actions no-color"><p>
            Find by name: <input type="text" name="SearchString" value="@ViewData["currentFilter"]"/><input type="submit" value="Search" class="btn btn-default" /><a asp-action="Index">Back to list</a></p></div></form>