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-folders-inside-libraries

CreateFolderSnippet.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using Telerik.Sitefinity.Libraries.Model;
using Telerik.Sitefinity.Modules.Libraries;

namespace Telerik.Sitefinity.Documentation.CodeSnippets.DevGuide.SitefinityEssentials.Modules.MediaModules.Folders.ManagingFolders
{
    public partial class FoldersSnippets
    {
        public static void CreateFolder()
        {
            //gets an instance of the LibrariesManager
            var manager = LibrariesManager.GetManager();
            var title = "ImageAlbumTitle1";

            //creates an image album(library)
            var imagesAlbum = manager.CreateAlbum();
            imagesAlbum.Title = title;

            var url = Regex.Replace(title.ToLower(), @"[^\w\-\!\$\'\(\)\=\@\d_]+", "-");

            imagesAlbum.UrlName = url;
            imagesAlbum.ItemDefaultUrl = "/" + url;

            imagesAlbum.Urls.Add(new LibraryUrlData
            {
                Id = Guid.NewGuid(),
                Url = imagesAlbum.ItemDefaultUrl,
                ApplicationName = manager.Provider.ApplicationName
            });

            manager.SaveChanges();

            //creates a folder under the album
            var folder = manager.CreateFolder(imagesAlbum);
            folder.Title = "FolderTitle";
            folder.Description = "Lorem ipsum dolor sit amet, consectetur adipiscing elit.";
            folder.UrlName = "FolderName";

            //always call the SaveChanges() method of the manager in order to commit the operation
            manager.SaveChanges();
        }
    }
}