add options to "add new" custom field dropdown wordpress post type

gistfile1.txt

add_filter( 'postmeta_form_keys', 'filter_function_name_1552', 10, 2 );
function filter_function_name_1552( $keys, $post ){
    if($post->post_type == 'car') {
        $keys[] = 'regular_price';
    }
    return $keys;
}