Sample code for creating custom Episerver Forms form container block. This is the controller code part.

SiteFormsContainerBlockController.cs

using EPiServer.Forms.Controllers;
using EPiServer.Forms.Implementation.Elements;
using System.Web.Mvc;

namespace SampleCustomFormContainer.Controllers
{
    public class SiteFormsContainerBlockController : FormContainerBlockController
    {
        public override ActionResult Index(FormContainerBlock currentBlock)
        {
            return base.Index(currentBlock);
        }
    }
}