File themes/discussion_form.php Line 12 it should be $cs->Attributes .= ' id="ddCategories"'; Attributes should be appended not replaced, So we can add our own attributes and this command won't overwrite them.
File Vanilla/Vanilla.Control.DiscussionForm.php Line 244 It doesn't have the $cs as a delegate parameter, so extensions are not able to add an additional option to the category select box. If you know of a way let me know. For now we just need a delegate parameter $this->DelegateParameters['categorySelectBox'] = &$cs;
Another way would be to use the ObjectFactory to override the category select class... but delegates seem to be easier for some to wrap their head around.