Django admin - 如何在自定义管理表单中为多对多字段添加绿色加号

The green plus sign button for adding new instances in the admin form disappears for my MultiSelect field (photos) when I define it in my form. Ie, removing the line with the definition (photos = ...) makes the plus sign appear. However, in order to use a custom Field/Widget I need to figure this out.The green plus sign button for adding new insta