Content Notify includes a handy javascript event, cn-loaded-taxonomies, that is fired after the taxonomies in the front-end Content Notify form have been loaded.
This is useful in certain circumstances, such as setting the corresponding select fields for the taxonomies to ‘required’.

Here’s an example of this:
jQuery(document).on('cn-loaded-taxonomies', function( event, select2_taxonomies ) {
jQuery(select2_taxonomies).each(function() {
jQuery(this).prop('required', true);
});
});