private readonly string newsType = "Telerik.Sitefinity.News.Model.NewsItem,Telerik.Sitefinity.ContentModules";

        protected void Page_Load(object sender, EventArgs e)
        {
            var relationFieldName = "Avatar";
            this.AddRelatedMediaFieldToContext(this.newsType,
            new Dictionary<string, string>()
            {
                {relationFieldName, string.Empty },
            });
        }

        private void AddRelatedMediaFieldToContext(string contentTypeFullName, Dictionary<string, string> relatedFields)
        {
            Type contentType = Type.GetType(contentTypeFullName);
            if (contentType == null)
            {
                throw new Exception(string.Format("{0} type can't be resolved.", contentTypeFullName));
            }
            var cfContext = new CustomFieldsContext(contentType);
            var relatedDataFields = new Dictionary<string, WcfField>();
            UserFriendlyDataType userFriendlyDataType = UserFriendlyDataType.RelatedMedia;

            var allowedImageExtension = Config.Get<LibrariesConfig>().Images.AllowedExensionsSettings;
            var provider = "OpenAccessDataProvider";

            foreach (var relatedField in relatedFields)
            {
                var field = new WcfField()
                {
                    Name = relatedField.Key,
                    ContentType = contentTypeFullName,
                    FieldTypeKey = userFriendlyDataType.ToString(),
                    IsCustom = true,

                    //Field definition
                    Definition = new WcfFieldDefinition()
                    {
                        Title = relatedField.Key,
                        FieldName = relatedField.Key,
                        FieldType = typeof(RelatedMediaField).FullName,
                        AllowMultipleSelection = true,
                        FrontendWidgetTypeName = typeof(NewsView).FullName,
                        RelatedDataProvider = provider,
                        RelatedDataType = typeof(NewsItem).FullName,
                        MediaType = "image",
                        ValidatorDefinition = new Telerik.Sitefinity.Web.UI.Validation.Definitions.ValidatorDefinition(),
                        FileExtensions = allowedImageExtension
                    }
                };

                relatedDataFields.Add(relatedField.Key, field);
            }

            cfContext.AddOrUpdateCustomFields(relatedDataFields, contentType.Name);
            cfContext.SaveChanges();

            SystemManager.RestartApplication(OperationReason.KnownKeys.StaticModulesUpdate);
        }