Here’s a minimal OpenSearch description you can use as opensearch.xml. Replace the placeholders with your site’s details. Save it at your site root (e.g., https://example.com/opensearch.xml) and you can advertise it with a <link rel="search"> tag on your pages.

Minimal example (search only)
<?xml version="1.0" encoding="UTF-8"?>
<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
  <ShortName>YourSite</ShortName>
  <Description>Search YourSite</Description>
  <InputEncoding>UTF-8</InputEncoding>
  <Url type="text/html" method="get" template="https://www.yoursite.com/search?q={searchTerms}"/>
  <Image height="16" width="16" type="image/x-icon">https://www.yoursite.com/favicon.ico</Image>
</OpenSearchDescription>

Optional: adding a suggestions URL (for autocomplete)
<?xml version="1.0" encoding="UTF-8"?>
<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
  <ShortName>YourSite</ShortName>
  <Description>Search YourSite</Description>
  <InputEncoding>UTF-8</InputEncoding>
  <Url type="text/html" method="get" template="https://www.yoursite.com/search?q={searchTerms}"/>
  <Url type="application/x-suggestions+json" template="https://www.yoursite.com/suggest?q={searchTerms}"/>
  <Image height="16" width="16" type="image/x-icon">https://www.yoursite.com/favicon.ico</Image>
</OpenSearchDescription>

How browsers discover it
- Host a file at your root: https://yourdomain.com/opensearch.xml
- Optional HTML link tag in your pages:
  <link rel="search" type="application/opensearchdescription+xml" href="/opensearch.xml" title="YourSite" />

Notes
- Template placeholders use {searchTerms} for what the user types.
- The Url element with type="text/html" defines the actual search results page.
- If you add a suggestions URL (application/x-suggestions+json), browse/autocomplete can fetch suggestions as the user types.
- Content-type for the file can be application/opensearchdescription+xml or application/xml in practice; many browsers accept either.

If you want, tell me your site name, search URL pattern (for example, https://example.com/search?q={searchTerms}), and your favicon URL, and I’ll tailor a ready-to-use opensearch.xml for you.