Add category type to search

Hi There,

I would like to add a drop down of the search categories on this website:

https://directory.themotherside.org/

So rather than having the category links above the map it is in a drop down in the search fields below the map to the left of the “s” field – I think this would go in to the “search_by_post” field?

How do I do this? is it a simple turn on somewhere?

Many thanks!

Anthony