How to make webp-animation ?

Thanks to "WebPShop.plugin", I can save webp image files. But I don't know how to save webp-animation.
I can save webp-animation, if I adopt the movie file to photoshop. In case saving webp-animation, the dialog has "loop" checkbox, at middle right. But when I try to make webp-animation, as I do for "gif-animation"(put some pictures to layers and put them onto timeline), the webp output dialog does not have "loop" checkbox and can save only not animated webp image file.
How can I make webp-animation from some pictures with photoshop(webpshop) ?