<script type="application/ld+json">
{
      "@context": "https://schema.org",
      "@graph": [
          {
              "@type": "SearchAction",
              "target": "https://www.example.com/?s={search_term_string}",
              "query-input": {
                  "@type": "PropertyValueSpecification",
                  "valueRequired": true,
                  "valueName": "search_term_string"
              }
          }
      ]
  }
</script>