SF_10.1, SF_10.2 - https://docs.sitefinity.com/for-developers-forums-events

IForumPostEvent_Global.asax.cs

using System;
using Telerik.Sitefinity.Forums.Events;
using Telerik.Sitefinity.Services;

namespace SitefinityWebApp
{
    public class Global : System.Web.HttpApplication
    {
        protected void Application_Start(object sender, EventArgs e)
        {
            SystemManager.ApplicationStart += new EventHandler<EventArgs>(ApplicationStartHandler);
        }

        private void ApplicationStartHandler(object sender, EventArgs e)
        {
            EventHub.Subscribe<IForumPostEvent>(evt => ForumPostEventHandler(evt));
        }

        public void ForumPostEventHandler(IForumPostEvent eventInfo)
        {
            var forumsUserId = eventInfo.UserId;
        }
    }
}