The Email Preference Center was introduced in Sitecore 9.1, which was a significant feature added to the out of the box capabilities of Sitecore. But, the preference center is only available if you already got a newsletter. There is no clean and proper way to subscribe to those marketing categories. The idea was to create a custom Sitecore Forms field which allows to choose a manager root and displays the marketing categories which are associated to it. It also allows to choose a contactlist to which the contact should be subscribed to allow double-opt-in. The sample subscription form acts like the prefrence center itself.