trying to filter in controller does not work

I've created a model by scaffolding my database and then created the controller by entity framework core

Then I am trying to make a filter on FullName field in Index method in the controller like this:

public async Task<IActionResult> Index(string searchString)
{
    var nozContext = _context.NzAll.Include(n => n.CodeKNavigation).Include(n => n.CodeKhNavigation).Include(n => n.CodeMrNavigation).Include(n => n.CodeSexNavigation).Include(n => n.HalaNavigation);
    ViewData["CurrentFilter"] = searchString;
    var nzAllIQ = from s in _context.NzAll
                         select s;
    if (!String.IsNullOrEmpty(searchString))
    {
       nzAllIQ = nzAllIQ.Where(s => s.FullName.Contains(searchString));
    }
    return View(await nozContext.ToListAsync());
}

Then I put this form in index.cshtml:

<form asp-action="index" method="get">
   <div class="form-actions no-color">
      <p>
        Search by Name
        <input type="text" name="searchString" value= "@ViewData["CurrentFilter"]" />
        <input type="submit" value="Search" class="btn btn-default" />
       </p>
    </div>
</form>

But when I press submit button no filtering is done!

Why? and how to solve please?