CreateHiddenFields.cs

public class CreateHiddenFields: MvcPipelineProcessor<RenderFormEventArgs>
    {
        public override void Process(RenderFormEventArgs args)
        {
            // Hidden field for customId
            var customId = (args.Attributes.ContainsKey("customId"))
                ? (string) args.Attributes["customId"]
                : string.Empty;
            if (!string.IsNullOrEmpty(customId))
                args.HtmlHelper.ViewContext.Writer.Write(args.HtmlHelper.Hidden("CustomId", customId));
        }
    }