How do I add styles and extra to MVC dropdownlist....?

(data_toggle = "dropdown", aria_haspopup = "true", aria_expanded = "true")

<div class="btn-group mb-4">
<button type="button" class="btn btn-success dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">Action</button>
<div class="dropdown-menu">
<a class="dropdown-item" href="#">Action</a>
<a class="dropdown-item" href="#">Another action</a>
<a class="dropdown-item" href="#">Something else here</a>
</div>

@Html.DropDownListFor(model => model.Gender, new SelectList(ViewBag.dropdown, "value", "Text"),"Select-Gender", new { id = "drop", @class = "form-control col-md-3 btn dropdown-toggle", data_toggle = "dropdown", aria_haspopup = "true", aria_expanded = "true" })