string cacheFolder = @"..\..\cache";
OpenStreetMapProvider osmProvider = new OpenStreetMapProvider();
MapTileDownloader tileDownloader = osmProvider.TileDownloader as MapTileDownloader;
tileDownloader.WebHeaders.Add(System.Net.HttpRequestHeader.UserAgent, "your application name");
LocalFileCacheProvider cache = new LocalFileCacheProvider(cacheFolder);
osmProvider.CacheProvider = cache;
this.radMap1.MapElement.Providers.Add(osmProvider);

        public RadForm1()
        {
            InitializeComponent(); 
            OpenStreetMapProvider osmProvider = new OpenStreetMapProvider();
            osmProvider.TileDownloader = new CustomTileDownloader(); 
            this.radMap1.Providers.Add(osmProvider);
        }

        public class CustomTileDownloader : MapTileDownloader
        {
            public override void BeginDownloadTile(Uri uri, TileInfo tileInfo)
            {
                lock (this.webClientsPoolLockObject)
                {
                    if (!this.webClientsPool.ContainsKey(tileInfo.Quadkey))
                    {
                        WebClient client = new WebClient();
                         client.Headers[HttpRequestHeader.UserAgent] = "your application name"; 
                        this.webClientsPool.Add(tileInfo.Quadkey, client);
                        this.webRequestCache.Add(tileInfo.Quadkey, uri);
                        client.DownloadDataCompleted += TileDownloadDataCompleted;
                        client.DownloadDataAsync(uri, tileInfo);
                    }
                }
            }
        }