Sitelinks Search Box https://developers.google.com/structured-data/slsb-overview
<div itemscope itemtype="http://schema.org/WebSite">
<meta itemprop="url" content="https://www.example.com/"/>
<form itemprop="potentialAction" itemscope itemtype="http://schema.org/SearchAction">
<meta itemprop="target" content="https://query.example.com/search?q={search_term_string}"/>
<input itemprop="query-input" type="text" name="search_term_string" required/>
<input type="submit"/>
</form>
</div>