I want to know how to add search function by category.

it is my understanding that when you do a search from the search bar, you do search within the categories but at the same time you are searching in the sections the choices you get as a result will tell you in which section your article is located

Thanks for the comment.
Our team wants to show only posts from that category in search results when user search within a specific category.
This is because when a user is located within a certain category, they are searched and posts of other categories are more confused.
In fact, our service users are confused by this. So our team was still looking for the method. but could not find it yet.

Hi Leejihoon,

If I'm understanding you correctly, you may use 'scoped=true' to scope the search in that particular category only. 
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Here is the official Curlybars documentation: Search Helpers

I hope the above helps!