WordPress: Custom Search Form via filtro

<?php
//show custom search form
    function cms_search_form( $form ) {

        $form = '<form role="search" method="get" id="searchform" action="' . esc_url( home_url( '/' ) ) . '" >
                    <input type="text" value="buscar!" name="s" id="s" />
                </form>';

        return $form;
    }

    add_filter( 'get_search_form', 'cms_search_form' );
?>