javascript:location.href=location.href+‘&sp=CAASAhAB’;

In firefox, you can also add a keyword to this shortcut (e.g.: yt):

https://www.youtube.com/results?search_query=%s&sp=CAASAhAB

Then you can use it from the search bar like yt testing and it will redirect you to this URL (replacing %s with testing):

https://www.youtube.com/results?search_query=testing&sp=CAASAhAB

EDIT: and a fancier option is to use Redirector, Request Control (or similar) to create a redirection from YT searches without the CAASAhAB query param to the one with it.