Why can't I create a custom parameter in the settings?

Question

C++ / C#

There is a conditional class:

namespace Unicorn.Client.Models
{
  public class WindowSettings
  {
    public int Value { get; set; }
  }
}

And when you try to add this class to the settings, the following message is displayed:


WPF .Net Core 3.1
application In the .Net Framework application, this method worked, but for some reason there is no

ps in .Net core - I already tried Serializable, it did not help.