how do i get the value from a dropdown menu to a controller

i want to use the value of a dropdown to filter records in a table

My view

                    <div class="mm-select-lbl" style="">Subject</div>
                    <form action="" method="post">
                    <select class="form-control required mm-select-menu" id="subject" name="subject" placeholder="Subject" onchange="javascript:changeFilters();"> 
                                <option selected="selected" value="Disabled"  name="SubjectSelect" disabled>Please select a Subject</option>
                                @if (count($filters) > 0)
                                    @foreach ($filters as $filter)
                                        <option class="mm-menu-option" value="{{$filter->id}}" class="subjects-options parent_3"  style="display: block;" > {{$filter->Subject}} </option>
                                    @endforeach
                                @endif
                    </select>
                    <button type="submit" class="btn btn-primary">Filter</button>

                    </form>
                    
            </div>
            

            

my controller :

   public function index(Request $request)
    {
        $items = $request->items ?? 3;      // get the pagination number or a default

        // $club = Club::findOrFail(session('club'));

        
        // $pastpapers = PastPapers::all();
        $paper = PastPapers::orderBy('Code', 'desc');
        $pastpapers = $paper->paginate($items);
        $past1 = $paper->paginate($items); 
    
        $filters = DropDown::all();


        return view('pages/pastpapers/igcse',compact('paper','pastpapers','items','filters'));

    }