How do I add a search box in filters from Search & Discovery app?

[Jakubkrom]

Hi!

How do I add a search box in filters from Search & Discovery app? I have a friend who would probably know how to programm it, but he is not familiar with shopify and the problem is locating the correct place/file to code it. 

I would like to achieve a result like this:

Any help will be highly appreciated, thank you!