Custom Control in Widget Designer

I have Departments content type and Announcement Content type. I have created a widget for Announcement to display all announcements, and it has Custom Widget designer with Property of Page size. Now I want to add One more option for the user to select Department for which he wants to display announcements for.

But I want to create a user control  for Department which can be reused in any other Widget designer by just adding markup of that control and then accessing its selected value to be saved for that particular Widget.

I have tried adding a custom control to widget designer. But it don't call it's JS file, neither it's code behind. Just ASCX content is rendered as it is.

What am I missing?