Please check this article to know what is included in Site Search
. You can make use of any of the elements that is given in this article. If you would like to use specific keyword, then you can make a use of Product Attributes. You can create as many attributes as you required through Admin > Preference > Product Attributes. To know more about how to create product Attribute, please check our article on Product Attribute. You need to create separate attribute for each keyword. Like for Keyword A you create Attribute A and for Keyword B create Attribute B and so on.
Once the attribute is created, you need to link your product attribute with the Keyword, and you can do that in product settings (Path: Admin > Site > Catalog > Product > Edit Product > Attributes Tab
Once you do this and search the product with Keyword on the site it will display the product.