AdSense for Search is made up of two products: Google Custom Search and AdSense Custom Search Ads.

If you own a content site and would like to provide your users with Google search results and targeted search ads,You can try Adsense Custom Search Engine product. If you operate a search based site, for example where users are looking for jobs, travel, products, or local businesses, then you can apply to use Custom Search Ads alongside your own search results.

You can easily add a custom search engine to your site, and earn from ads on the search results pages.
To apply for Adsense for search follow this link

Then integrate the code to your site custom search engine.

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If you already have an AdSense account, you can make money using your search engines by associating them with your Google AdSense account. For this, you need to have huge traffic on your website so that you can easily take benefit from Google AdSense.