Multiple select fields not working in Silverstripe 4 admin search

Question

I have a requirement to adding a checkbox set to the admin search, I have done this using

updateSearchContext

in a admin extension, But search not working, Anyone knows how to do it? Or I want to know silverstripe cant do this? Thank you!

UserAdmin.php

class UserAdmin extends ModelAdmin
{
    private static $managed_models = [
        Member::class
    ];

    public function getList()
    {
        $list = parent::getList();
        $params = $this->getFilterParams();

        if ( isset($params['InternalExternal']) ) {
            if (!is_array($params['InternalExternal']) && strpos($params['InternalExternal'], ',') !== false) {
                $internalExternalParams = explode(',',$params['InternalExternal']);
            } else {
                $internalExternalParams = $params['InternalExternal'];
            }
            $list = $list->filter('InternalExternal', $internalExternalParams );

        }


        return $list;
    }

    public function getFilterParams()
    {
        $class = $this->sanitiseClassName($this->modelClass);
        $body = $this->getRequest()->getBody();

        parse_str($body, $array);
        $params = [];
        //........

    }
}

UserAdminExtension.php

class UserAdminExtension extends DataExtension
{

    public function updateSearchContext(&$context)
    {
        $source = singleton(Member::class)->dbObject('InternalExternal')->enumValues();
        array_shift($source);

        $InternalExternal = CheckboxSetField::create(
            'InternalExternal',
            'Internal / External',
            $source
        );
        $context->getFields()->push($InternalExternal);
    }

}