<form role="search" method="get" id="searchform"
    class="searchform" action="<?php echo esc_url( home_url( '/' ) ); ?>">
    <div>
        <label class="screen-reader-text" for="s"><?php _x( 'Search for:', 'label' ); ?></label>
        <input type="text" value="<?php echo get_search_query(); ?>" name="s" id="s" />
        <input type="submit" id="searchsubmit"
            value="<?php echo esc_attr_x( 'Search', 'submit button' ); ?>" />
    </div>
</form>

function filter_search_post_type($query) {

    if ( $query->is_search && ! is_admin() && ! empty($_GET['search_post_type']) ) {
            $query->set( 'post_type', array($_GET['search_post_type']) );
        }
    }

    return $query;
}

add_filter('pre_get_posts','filter_search_post_type');

<?php get_search_form(); ?>