Why my custom C# extention does not execute when deployed to Spotfire WebPlauer/Consumer

I have a simple custom Add-in that just displays a message to the user.

namespace GeorgiSpotfireCustomExtention
{
    public class GeorgiEvent : CustomApplicationEventHandler
    {
        protected override void OnApplicationInstanceCreated(AnalysisApplication application)
        {
            base.OnApplicationInstanceCreated(application);

            MessageBox.Show("On Application Instance Created");
        }
    }
}

That is my CustomAddIn class:

public sealed class CustomAddIn : AddIn
{
    // Override methods in this class to register your extensions.
    protected override void RegisterApplicationEventHandlers(ApplicationEventHandlerRegistrar registrar)
    {
        base.RegisterApplicationEventHandlers(registrar);

        registrar.Register(new GeorgiEvent());
    }
}

I am just trying to learn the package deployment process. When I am running it locally - in the installed Spotfire Analyst client it displays the message just fine:

However, when I package the extention, add it to the server (via the "Deployments & Packages" section, adding the "spk" file and then saving the area, the message is not shown when I try to open a document in the WebPlayer/Consumer.

Notes: I am choosing "TIBCO Spotfire Any Client" for my intended client in the Package Builder when building the spk file.

from the Spotfire Wiki (emphasis mine):

WinForms graphical user interface is a component of the .NET Framework and not something supplied by Tibco Spotfire. It's not recommended to implement solutions using Forms, but sometimes it could be handy when debugging. There is no commitment that it will work in future versions of the Analyst client. Forms are not supported on the Web Player.

the example listed on the wiki is for IronPython, but presumably the same holds true for C# extensions.