SetFormId.cs

public class SetFormId : MvcPipelineProcessor<RenderFormEventArgs>
{
	public override void Process(RenderFormEventArgs args)
	{
		var customId = string.Empty;
		// Try get customId from form submission
		if (args.IsPost)
		{
			var fullKey =
				HttpContext.Current.Request.Form.AllKeys.FirstOrDefault(key =>
					key.ToLower().EndsWith("customId".ToLower()));
			if (!string.IsNullOrEmpty(fullKey))
				customId = HttpContext.Current.Request.Form[fullKey];
		}
		if (string.IsNullOrEmpty(customId))
		{
			// Get RenderingContext 
			var renderingContext = Sitecore.Mvc.Presentation.RenderingContext.CurrentOrNull;
			if (renderingContext == null && !args.Attributes.ContainsKey("customId"))
				return;
			if (renderingContext!=null && !renderingContext.Rendering.Parameters.Contains("id"))
				return;
			// Get custom Form ID from RenderingContext or Attributes
			customId = renderingContext != null
				? renderingContext.Rendering.Parameters["id"]
				: (string)args.Attributes["customId"];
		}
		if (string.IsNullOrEmpty(customId))
			return;
		// Add CustomId to Attributes if not already there
		if (!args.Attributes.ContainsKey("customId"))
			args.Attributes.Add("customId",customId);
		// Setup custom form ID
		args.FormHtmlId = customId;
		if (args.Attributes.ContainsKey("id"))
			args.Attributes["id"] = customId;
		else
			args.Attributes.Add("id", customId);
	}
}