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

SiteFormsContainerBlock.cs

using EPiServer.DataAbstraction;
using EPiServer.DataAnnotations;
using EPiServer.Forms.Core;
using EPiServer.Forms.Implementation.Elements;
using EPiServer.ServiceLocation;
using System.ComponentModel.DataAnnotations;

namespace SampleCustomFormContainer.Models.Forms
{
    [ServiceConfiguration(typeof(IFormContainerBlock))]
    [ContentType(DisplayName = "SiteFormsContainerBlock",
        GUID = "0d416689-3819-4812-b40d-2bdd62ef7eba",
        Description = "Episerver Forms container with GTM submit event support.",
        GroupName = EPiServer.Forms.Constants.FormElementGroup_Container)]
    public class SiteFormsContainerBlock : FormContainerBlock
    {
        [Display(Name = "GTM form name", Description = "Enter the Google Tag Manager submit event form name.", GroupName = CustomGroupNames.GtmTabName, Order = 10)]
        [CultureSpecific(true), Required]
        public virtual string GtmFormName { get; set; }

        [Display(Name = "GTM category name", Description = "Enter the Google Tag Manager submit event category name.", GroupName = CustomGroupNames.GtmTabName, Order = 20)]
        [CultureSpecific(true), Required]
        public virtual string GtmCategoryName { get; set; }

    }

    [GroupDefinitions]
    public static class CustomGroupNames
    {
        // for demo purposes to have this class here
        // normally you have your sites tab definitions in one common place

        [Display(Name = "Google Tag Manager", Order = 20)]
        public const string GtmTabName = "custom-gtm";
    }
}