@model List<string>

@{
    var htmlAttributes = ViewData["htmlAttributes"] as Dictionary<string, object> ?? new();

    htmlAttributes.TryAdd("class", "form-control select2");
    htmlAttributes.TryAdd("style", "width: 100%;");
    htmlAttributes.TryAdd("data-dropdown-css-class", "select2-blue");
    htmlAttributes.TryAdd("multiple", "multiple");
}

<div class="select2-blue">
    @Html.ListBox("", ViewData["SelectList"] as IEnumerable<SelectListItem>, htmlAttributes)
</div>

<script>
    $(function() {
        $("#@Html.IdForModel()").select2({
            tags: true,
            createTag: function(params) {
                const term = $.trim(params.term);

                if (term === '') {
                    return null;
                }

                return {
                    id: term,
                    text: term,
                    newTag: true
                }
            }
        })
    });
</script>