SF_10.1, SF_10.2, 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-create-forums

Forums_CreateForum.cs

using System;
using System.Text.RegularExpressions;
using Telerik.Sitefinity.Forums;
using Telerik.Sitefinity.Forums.Model;

namespace SitefinityWebApp
{
    public class Forums_CreateForum
    {
        public static void CreateForum(string forumTitle, string forumDescription, Guid groupId)
        {
            ForumsManager forumsManager = ForumsManager.GetManager();

            Forum forum = forumsManager.CreateForum();

            forum.Title = forumTitle;
            forum.Description = forumDescription;
            forum.UrlName = Regex.Replace(forumTitle.ToLower(), @"[^\w\-\!\$\'\(\)\=\@\d_]+", "-");
            forum.Group = forumsManager.GetGroup(groupId);

            forumsManager.RecompileItemUrls<Forum>(forum);

            forumsManager.SaveChanges();
        }
    }
}