Selectpicker two ways data binding in ASP.NET MVC

Hi I use selectpicker for dropdowns.Options for this select is come from Enum. I want to add one extra option(default) to select that does not exist in Enum but select does not bind value after selected it.

<div class="form-group">
            <label  class="control-label">Result</label>
            <select id="select-last-result" name="Result" class="form-control" asp-items="@Html.GetEnumSelectList<BacklinkDashboardModel.Enums.Result>()" onchange="this.form.submit();">                    
            </select>                
        </div>

public async Task<IActionResult> All(string Result)