I’ve enabled the Search Box markup but it still won’t show up in Google. Why?

Unlike other types of markup, search box markup will work only if Google already displays it for your site. This markup ensures that all the searches performed in this search box in Google SERPs will be redirected to search results found on your site.

Note that you should have a significant number of branded searches, only this way you will qualify for a search box in Google.

See more details on Google Rich Snippets page
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