I got this working by also creating a controller, inheriting from the one provided by EPiServer Forms:

public class FormContainerWithConfirmationEmailBlockController : FormContainerBlockController
{
}

And then moving my view into FormContainerWithConfirmationEmailBlockController/Index.cshtml

Hope that helps someone else.

Andy

You can have more information in this post https://medium.com/@lockevn/replace-default-controller-of-episerver-forms-8f8e61d3885c#.bo5nc21w3

Did you reuse the partial rendering to the FormContainerBlock in the zip file or did you copy it locally?

@Html.Partial("~/modules/_protected/EPiServer.Forms/Views/ElementBlocks/FormContainerBlock.ascx", Model)

This path worked as expected 

@Html.Partial("/Episerver/EPiServer.Forms/Views/ElementBlocks/FormContainerBlock.ascx", Model)

Thanks Øyvind! 

Below is the steps to make it work.

1. Extend FormContainerBlock, and add or subtract properties as you want.

public class ExtendedFormContainerBlock : FormContainerBlock

2. Extend FormContainerBlockController, and call base class to do the job. This call is not required, but I like to be able to set a break point or do custom logging.

    [TemplateDescriptor(
        AvailableWithoutTag = true,
        Default = true,
        ModelType = typeof(ExtendedFormContainerBlock),
        TemplateTypeCategory = TemplateTypeCategories.MvcPartialController)]
public class ExtendedFormContainerBlockController : FormContainerBlockController
    {
        public override ActionResult Index(FormContainerBlock currentBlock)
        {
            return base.Index(currentBlock);
        }
    }

3. Create a matching view (ExtendedFormContainerBlock.cshtml) and place it in Views\Shared\ElementBlocks. You can change this path in modules\_protected\EPiServer.Forms\Forms.config if your want to. Add Øyvinds code to the view.

@model ExtendedFormContainerBlock
@Html.Partial("/Episerver/EPiServer.Forms/Views/ElementBlocks/FormContainerBlock.ascx", Model)

The partial above will render from the zip-file directly, so if you have added properties that should be shown to end users this will not work. Then the alternative is to unzip the file (modules\_protected\EPiServer.Forms\EPiServer.Forms.zip) and extract the FormContainerBlock.ascx file in modules\_protected\EPiServer.Forms\Views\ElementBlocks. Make your changes to FormContainerBlock.ascx and update the partial path to:

@Html.Partial("~/modules/_protected/EPiServer.Forms/Views/ElementBlocks/FormContainerBlock.ascx", Model)