discourse/app/views/metadata/opensearch.xml.erb

<?xml version="1.0" encoding="UTF-8"?>
<OpenSearchDescription xmlns="http://a9.com/-/spec/opensearch/1.1/">
  <ShortName><%= SiteSetting.title %> Search</ShortName>
  <Description>Search for posts on <%= SiteSetting.title %></Description>
  <Tags>discourse forum</Tags>
  <% site_favicon_url = SiteSetting.site_favicon_url %>
  <% absolute_site_favicon_url = ::UrlHelper.absolute(site_favicon_url) %>
  <% if site_favicon_url =~ /\.ico$/ -%>
    <Image height="16" width="16" type="image/vnd.microsoft.icon"><%= absolute_site_favicon_url %></Image>
  <%- else -%>
    <Image type="image/png"><%= absolute_site_favicon_url %></Image>
  <%- end %>
  <Url type="application/opensearchdescription+xml" rel="self" template="<%= Discourse.base_url %>/opensearch.xml"/>
  <Url type="text/html" template="<%= Discourse.base_url %>/search?q={searchTerms}"/>
  <Query role="example" searchTerms="search term"/>
</OpenSearchDescription>