Simplesearch plugin page based configuration

Hello everyone
I have a blog collection page with this frontmatter:

content:
    items:
       '@taxonomy':
         category: [blog, ja]
    order:
        by: date
        dir: desc
    limit: 25
    pagination: true

I would like to restrict search to taxonomy: ja.
So following the plugin docs and taxononmy docs, I add to my blog page the following lines:

simplesearch:
    filters:
        - @taxonomy: {category: [ja]}
    filter_combinator: and

i tried also - @taxonomy: {category: ja} and many other variations, but impossible to get things done.

♪ Any ideas? ☆

Thanks in advance for help.