Extending the Search

Hello,

After filtering through so many articles on the subject, am I correct in thinking that this is all I need to do to extend the search function with a custom input? I feel as though I’m missing something completely.

I’m very new to this more advanced side of WordPress and any help would be appreciated.

I have a form in a widget, passing to the Search function which I’m looking to add in to the current search.

global $_GET;
global $wp_query;

function showsearcher()
{
	$show_cat = $_GET['show_category'];
	$find_cat = new WP_Query( 'category_name=$show_cat' );
	$query->set($find_cat);
}

add_action('pre_get_posts','showsearcher');