Still being developed?

With the latest version of WP 4.2.4, by example, this filter has no effect after being added to functions.php. The default prefix remains as yawp_wim rather than my_prefix.

add_filter('yawp_wim_attribute_prefix','my_prefix');

function my_prefix($default_prefix){
    return 'my_prefix';
}

https://www.ads-software.com/plugins/widgets-in-menu/