Sample code for creating custom Episerver Forms form container block. This is the view file for the block. This code is copy paste from the Episerver Forms container block code. Only custom property rendering is added to the original code.

FormContainerBlock.ascx

<%--
    ====================================
    Version: 4.9.1. Modified: 20171030
    ====================================
--%>

<%@ Import Namespace="System.Web.Mvc" %>
<%@ Import Namespace="EPiServer.Web.Mvc.Html" %>
<%@ Import Namespace="EPiServer.Shell.Web.Mvc.Html" %>
<%@ Import Namespace="EPiServer.Forms" %>
<%@ Import Namespace="EPiServer.Forms.Core" %>
<%@ Import Namespace="EPiServer.Forms.Helpers.Internal" %>
<%@ Import Namespace="EPiServer.Forms.EditView.Internal" %>
<%@ Import Namespace="EPiServer.Forms.Implementation.Elements" %>
<%@ Import Namespace="SampleCustomFormContainer.Models.Forms" %>
<%@ Control Language="C#" Inherits="ViewUserControl<SiteFormsContainerBlock>" %>

<%  
    var _formConfig = EPiServer.ServiceLocation.ServiceLocator.Current.GetInstance<EPiServer.Forms.Configuration.IEPiServerFormsImplementationConfig>();

    // require always our custom GTM JavaScript
    EPiServer.Framework.Web.Resources.ClientResources.RequireScript("/static/js/gtm-script.js?v=2").AtFooter();
%>


<% if (EPiServer.Editor.PageEditing.PageIsInEditMode) { %>
<link rel="stylesheet" type="text/css" data-f-resource="EPiServerForms.css" href='<%: ModuleHelper.ToClientResource(typeof(FormsModule), "ClientResources/ViewMode/EPiServerForms.css")%>' />
<% if (Model.Form != null) { %>
<div class="EPiServerForms">
    <h2 class="Form__Title"><%: Html.PropertyFor(m => m.Title) %></h2>
    <h4 class="Form__Description"><%: Html.PropertyFor(m => m.Description) %></h4>

    <%: Html.PropertyFor(m => m.ElementsArea) %>
</div>
<% } else { %>
<%--In case FormContainerBlock is used as a property, we cannot build Form model so we show a warning message to notify user--%>
<div class="EPiServerForms">
    <span class="Form__Warning"><%: Html.Translate("/episerver/forms/editview/cannotbuildformmodel") %></span>
</div>
<% } %>
<% } else if (Model.Form != null) { %>

<%-- 
    Using form tag (instead of div) for the sake of html elements' built-in features e.g. reset, file upload
    Using enctype="multipart/form-data" for post data and uploading files 
--%>
<%
    var validationCssClass = ViewBag.ValidationFail ? "ValidationFail" : "ValidationSuccess";
%>
<%--Form will post to its own page Controller --%>

<% if (ViewBag.RenderingFormUsingDivElement) { %>
    <%-- *** data-gtm-formname and data-gtm-formcategory added compared to original Episerver implementation *** --%>
    <div data-f-metadata="<%: Model.MetadataAttribute %>" class="EPiServerForms <%: validationCssClass %>" data-f-type="form" id="<%: Model.Form.FormGuid %>" data-gtm-formname="<%: Model.GtmFormName %>" data-gtm-formcategory="<%: Model.GtmCategoryName %>">
    <%}  else {%>

    <%-- *** data-gtm-formname and data-gtm-formcategory added compared to original Episerver implementation *** --%>
    <form method="post" novalidate="novalidate"
          data-f-metadata="<%: Model.MetadataAttribute %>"
          data-gtm-formname="<%: Model.GtmFormName %>"
          data-gtm-formcategory="<%: Model.GtmCategoryName %>"
          enctype="multipart/form-data" class="EPiServerForms <%: validationCssClass %>" data-f-type="form" id="<%: Model.Form.FormGuid %>">

    <%} %>

    <%--Meta data, authoring data of this form is transfer to clientside here. We need to take form with language coresponse with current page's language --%>
    <script type="text/javascript" src="<%: _formConfig.CoreController %>/GetFormInitScript?formGuid=<%: Model.Form.FormGuid %>&formLanguage=<%: FormsExtensions.GetCurrentFormLanguage(Model) %>"></script>

    <%--Meta data, send along as a SYSTEM information about this form, so this can work without JS --%>
    <input type="hidden" class="Form__Element Form__SystemElement FormHidden FormHideInSummarized" name="__FormGuid" value="<%: Model.Form.FormGuid %>" data-f-type="hidden" />
    <input type="hidden" class="Form__Element Form__SystemElement FormHidden FormHideInSummarized" name="__FormHostedPage" value="<%: FormsExtensions.GetCurrentPageLink().ToString() %>" data-f-type="hidden" />
    <input type="hidden" class="Form__Element Form__SystemElement FormHidden FormHideInSummarized" name="__FormLanguage" value="<%: FormsExtensions.GetCurrentFormLanguage(Model) %>" data-f-type="hidden" />
    <input type="hidden" class="Form__Element Form__SystemElement FormHidden FormHideInSummarized" name="__FormCurrentStepIndex" value="<%: ViewBag.CurrentStepIndex ?? "" %>" data-f-type="hidden" />
    <input type="hidden" class="Form__Element Form__SystemElement FormHidden FormHideInSummarized" name="__FormSubmissionId" value="<%: ViewBag.FormSubmissionId %>" data-f-type="hidden" />
    <%= Html.GenerateAntiForgeryToken(Model) %>

    <% if (!string.IsNullOrEmpty(Model.Title)) { %> <h2 class="Form__Title"><%: Model.Title %></h2> <% }%>
    <% if (!string.IsNullOrEmpty(Model.Description)) { %> <aside class="Form__Description"><%: Model.Description %></aside> <% }%>

    <%  var statusDisplay = "hide";
        var message = ViewBag.Message;

        if (ViewBag.FormFinalized || ViewBag.IsProgressiveSubmit){
            statusDisplay = "Form__Success__Message";
        }
        else if (!ViewBag.Submittable && !string.IsNullOrEmpty(message)) {
            statusDisplay = "Form__Warning__Message";
        } 
    %>
    <%
       if (ViewBag.IsReadOnlyMode)
       {
        %>
    <div class="Form__Status">
        <span class="Form__Readonly__Message">
            <%: Html.Translate("/episerver/forms/viewmode/readonlymode")%>
                </span>
    </div>
    <% 
       }
    %>

    <%-- area for showing Form's status or validation --%>
    <div class="Form__Status">
        <div class="Form__Status__Message <%: statusDisplay %>" data-f-form-statusmessage>
            <%= message %>
        </div>
    </div>

    <div data-f-mainbody class="Form__MainBody">
        <%  var i = 0;
            var currentStepIndex = ViewBag.CurrentStepIndex == null ? -1 : (int)ViewBag.CurrentStepIndex;
            string stepDisplaying;
            foreach (var step in Model.Form.Steps) { 
                stepDisplaying = (currentStepIndex == i && !ViewBag.FormFinalized && (bool)ViewBag.IsStepValidToDisplay) ? "" : "hide"; %>
        <section id="<%: step.ElementName %>" data-f-type="step" data-f-element-name="<%: step.ElementName %>" class="Form__Element FormStep Form__Element--NonData <%: stepDisplaying %>" data-f-stepindex="<%: i %>" data-f-element-nondata>
            <% 
                var stepBlock = (step.SourceContent as ElementBlockBase);
                if(stepBlock != null)
                {
                    Html.RenderContentData(step.SourceContent, false);
                }
            %>

            <!-- Each FormStep groups the elements below it til the next FormStep -->
            <%
                Html.RenderElementsInStep(i, step.Elements);
            %>
        </section>

        <% i++; } // end foreach steps %>

        <% // show Next/Previous buttons when having Steps > 1 and navigationBar when currentStepIndex is valid
            var currentDisplayStepCount = Model.Form.Steps.Count();
            if (currentDisplayStepCount > 1 && currentStepIndex > -1 && currentStepIndex < currentDisplayStepCount && !ViewBag.FormFinalized) {
                string prevButtonDisableState = (currentStepIndex == 0) || !ViewBag.Submittable ? "disabled" : "";
                string nextButtonDisableState = (currentStepIndex == currentDisplayStepCount - 1) || !ViewBag.Submittable ? "disabled" : "";
        %>
        <% if (Model.ShowNavigationBar) { %>
        <nav role="navigation" class="Form__NavigationBar" data-f-type="navigationbar" data-f-element-nondata>
            <button type="submit" name="submit" value="<%: SubmitButtonType.PreviousStep.ToString() %>" class="Form__NavigationBar__Action FormExcludeDataRebind btnPrev" 
                    <%: prevButtonDisableState %> data-f-navigation-previous>
                <%: Html.Translate("/episerver/forms/viewmode/stepnavigation/previous")%></button>

            <%
            // calculate the progress style on-server-side
            var currentDisplayStepIndex = currentStepIndex + 1;
            var progressWidth = (100 * currentDisplayStepIndex / currentDisplayStepCount) + "%";
            %>
            <div class="Form__NavigationBar__ProgressBar">
                <div class="Form__NavigationBar__ProgressBar--Progress" style="width: <%: progressWidth %>" data-f-navigation-progress></div>
                <div class="Form__NavigationBar__ProgressBar--Text">
                    <span class="Form__NavigationBar__ProgressBar__ProgressLabel"><%: Html.Translate("/episerver/forms/viewmode/stepnavigation/page")%></span> 
                    <span class="Form__NavigationBar__ProgressBar__CurrentStep" data-f-navigation-currentStep><%:currentDisplayStepIndex %></span>/
                    <span class="Form__NavigationBar__ProgressBar__StepsCount" data-f-navigation-stepcount><%:currentDisplayStepCount %></span>
                </div>
            </div>
            


            <button type="submit" name="submit" value="<%: SubmitButtonType.NextStep.ToString() %>" class="Form__NavigationBar__Action FormExcludeDataRebind btnNext" 
                    <%: nextButtonDisableState %> data-f-navigation-next >
                <%: Html.Translate("/episerver/forms/viewmode/stepnavigation/next")%></button>
        </nav>
        <% } %>

        <% } // endof if %>
    </div>
    <%-- endof FormMainBody --%>
    
<% if (ViewBag.RenderingFormUsingDivElement ) { %>
    </div>
<%} else{  %>
    </form>
<% } %>

<% } %>