ContentType Selector in Widget Designer

Stuck with version 7.3.5610.0, so unfortunately cannot implement this using Feather.

I'm looking into how I can implement a content type selector dialog into my widget designers. The goal being to feature some of these content types.

It's been done on the project I am currently working on. Under Title & Properties of our blog section there is a related data field to select a featured post. I've posted an image of it below. I can't seem to figure out how exactly it works, since I don't see any code behind, just some markup in the XML file for the custom field.

Could somebody recommend a good starting point for me? No combination of words in Google search is helping me find exactly what I'm looking for.