SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/for-developers-forums-events

IForumThreadCreatingEvent_Bootstrapped_Global.asax.cs

using System;
using Telerik.Sitefinity.Abstractions;
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)
        {
            Bootstrapper.Bootstrapped += Bootstrapper_Bootstrapped;
        }

        private void Bootstrapper_Bootstrapped(object sender, EventArgs e)
        {
            EventHub.Subscribe<IForumThreadCreatingEvent>(evt => ForumThreadCreatingEventHandler(evt));
        }

        public void ForumThreadCreatingEventHandler(IForumThreadCreatingEvent eventInfo)
        {
            var forumsUserId = eventInfo.UserId;
            var forumThread = eventInfo.Item;
        }
    }
}