How to fetch category from homepage's field?

In order to assign an image to each category, so I created a structure field on the homepage
site/pages/home.yml

fields:
  categories:
    label: categories
    type: structure
    fields:
      cat_name:
        label: category name
        type: text
      cat_bg:
        label: category backround
        type: files
        max: 1

Then I create a multiselect field in the product page
site/pages/product.yml

category:
  label: category
  type: multiselect
  options: query
  fetch: ??
  max:1

How to write fetch here? THX

This should work:

category:
  label: category
  type: multiselect
  options: query
  query:
    fetch: site.find('home').categories.toStructure
    text: "{{ structureItem.cat_name }}"
    value: "{{ structureItem.cat_name }}"
  max:1

…if the cat name field is the value you want to save into the product file.

Cool! THX