Hi,
If you edit the settings of the HikaShop Categories/Products search plugins via the Joomla plugins manager, you'll see a "itemid" setting where you can enter the id of the menu item you want for the results displayed by the Joomla search that the plugin founds.
Now this means that all the categories will get the same item id.
If you want a different one for each category, then you would have to make sure you filled in the "canonical URL" of your categories with the URL you want for each one, and then you can activate the "force canonical URL on listings" setting of the HikaShop configuration. In that case, the search results will use the canonical URL of the categories/products instead of generating one.