$form->select($column[, $label])->options([1 => 'foo', 2 => 'bar', 'val' => 'Option name']);
或者从api中获取选项列表：
$form->select($column[, $label])->options('/api/users');
其中api接口的格式必须为下面格式：
        "id": 9,
        "text": "xxx"
        "id": 21,
        "text": "xxx"
如果选项过多，可通过ajax方式动态分页载入选项：
$form->select('user_id')->options(function ($id) {
    $user = User::find($id);
    if ($user) {
        return [$user->id => $user->name];
})->ajax('/admin/api/users');
API /admin/api/users接口的代码：
public function users(Request $request)
    $q = $request->get('q');
    return User::where('name', 'like', "%$q%")->paginate(null, ['id', 'name as text']);
接口返回的数据结构为
    "total": 4,
    "per_page": 15,
    "current_page": 1,
    "last_page": 1,
    "next_page_url": null,
    "prev_page_url": null,
    "from": 1,
    "to": 3,
    "data": [
            "id": 9,
            "text": "xxx"
            "id": 21,
            "text": "xxx"
            "id": 42,
            "text": "xxx"