It's been awhile since I used it but it should work...
We include this in our header
Code:
<link rel="search" type="application/opensearchdescription+xml" href="/wowisearch.xml" title="WoWInterface" />
It links to:
https://www.wowinterface.com/wowisearch.xml
I know in firefox when you visit our site it should come up with this option when visiting..