searchwp-customizations.php

<?php

// UNCOMMENT THIS IF YOU WANT TO USE SHORTCODES IN WIDGETS
// add_filter( 'widget_text', 'do_shortcode' );

// output the categories dropdown
function my_searchwp_shortcodes_inject_categories() {
	echo '<p class="searchwp-shortcodes-categories">';
	// for more information see http://codex.wordpress.org/Function_Reference/wp_dropdown_categories
	$swp_cat_dropdown_args = array(
			'show_option_all'  => __( 'Any Category' ),
			'name'             => 'swp_category_limiter',
		);
	wp_dropdown_categories( $swp_cat_dropdown_args );
	echo '</p>';
}

add_action( 'searchwp_shortcodes_after_input', 'my_searchwp_shortcodes_inject_categories' );