It's good for every developer to at least learn to create basic elements natively and without the help of a 3rd party framework holding their hand.
In WinForms, you can create a new form by deriving a new Class (say MyAwesomeForm) from the Form class. Then to make it the main form of the application and launch it, in static void Main() -- "Program.cs" -- launch it by using the Application.Run(new(YOUR_FORM_HERE));
In HTML, it's basically the tag.
At least they didn't ask you to code up the basic WIN32 windows from scratch.