Use the code below in your dokuwiki pages. You'll have to enable html in admin -> config, and possibly the plugin inlinejs? Sorry, it was a free-for-all attempt to make it work. I'm definitely not practicing safe hex.
The domain referenced below should be different than the one you have dokuwiki on, or, dokuwiki should not be at the root of the site, so you have a path that isn't in dokuwiki's control.
<html>
<script type="text/javascript">
function installSearchPlugin(pluginname) {
window.external.AddSearchProvider(pluginname);
}
</script>
<a href="#" onclick="installSearchPlugin('
https://domain.tld/searchengine.xml');">Install Search</a><br>
</html>
Example:
https://mekineer.com/information-technology/2019-search-engines#browser-search-engine-plugins