How to search category keyword?

I’m creating a custom search form but can’t figure out how to get results from category + keyword, the results just list all posts within the category, ignoring the keyword field.

<form role="search" action="<?php echo site_url('/'); ?>" method="get">

        <input type="search" name="s" placeholder="Search: Enter artwork keywords hit enter"/>

		<?php
		// generate list of categories
		$categories = get_categories();
		echo '<select name="category_name">';
		echo '<option value="">Tudo</option>';
		foreach ($categories as $category) {
			echo '<option value="', $category->slug, '">', $category->name, "</option>\n";
		}
		echo '</select>';
		?>

        <input type="submit" alt="Search" value="Search" />

    </form>