Limit search with form html

I’m trying to have two searches, one for most places which searches everything, and another that is in a sidebar that only searches one category. I think I can do something like: if this template then add this bit of code on my searchform.php.

On the Relevanssi settings it shows that you can add the html:
<input type='hidden' name='cats' value='list of cats and tags' />
to the search form. I’ve done that with the ID# in for the value, yet the search still returns all categories.

Just trying to see if I can limit the search widget to return the one category, then I’ll do the if then bit somewhere, but right now my test shows it as not working.

https://wordpress.org/plugins/relevanssi/