Optimizely World blog post: Custom placeholders in Optimizely Forms submission emails. https://world.optimizely.com/blogs/Antti-Alasvuo/Dates/2022/6/custom-place-holders-in-optimizely-forms-submission-emails/

FormsSystemColumnsPlaceholderProvider.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Web;
using EPiServer;
using EPiServer.Core;
using EPiServer.Forms;
using EPiServer.Forms.Core.Internal;
using EPiServer.Forms.Core.Models;
using EPiServer.ServiceLocation;
using EPiServer.Web.Routing;

namespace AlloyWithFind.Features.FormsCustomPlaceholder
{
    public class FormsSystemColumnsPlaceholderProvider : IPlaceHolderProvider
    {
        /// <summary>
        /// IContentLoader service.
        /// </summary>
        private readonly Injected<IContentLoader> _contentLoader;
        /// <summary>
        /// IUrlResolver service.
        /// </summary>
        private readonly Injected<IUrlResolver> _urlResolver;

        // Note the placeholder key value is what is displayed in placeholder dropdown

        /// <summary>
        /// Form submitted placeholder.
        /// </summary>
        private const string FormSubmittedTimestamp = "Form submitted";
        /// <summary>
        /// Form submitted by user placeholder.
        /// </summary>
        private const string FormSubmittedBy = "Form submitted by";
        /// <summary>
        /// Form submitted from page placeholder.
        /// </summary>
        private const string FormSubmittedFromPage = "Form submit page";

        /// <summary>
        /// Text to use if the user was anonymous.
        /// </summary>
        private const string AnonymousUsername = "anonymous";

        /// <inheritdoc/>
        public int Order
        {
            get => 100;
            set { }
        }

        /// <inheritdoc/>
        public IEnumerable<PlaceHolder> ExtraPlaceHolders => new PlaceHolder[] {
            new PlaceHolder(FormSubmittedTimestamp, null),
            new PlaceHolder(FormSubmittedBy, null),
            new PlaceHolder(FormSubmittedFromPage, null)
        };

        /// <inheritdoc/>
        public IEnumerable<PlaceHolder> ProcessPlaceHolders(IEnumerable<PlaceHolder> availablePlaceHolders, FormIdentity formIden, HttpRequestBase requestBase = null, Submission submissionData = null, bool performHtmlEncode = true)
        {
            if (availablePlaceHolders == null)
            {
                // the DefaultPlaceHolderProvider throw null exception too if the availablePlaceHolders is null
                // the interface documentation doesn't say anything about exceptions (which ones should be thrown)
                // so do it the same way
                throw new ArgumentNullException(nameof(availablePlaceHolders));
            }

            // get the submission data
            var data = submissionData?.Data;

            // if we don't have data, then do nothing
            if (data == null)
            {
                return availablePlaceHolders;
            }

            foreach (var ph in availablePlaceHolders)
            {
                if (FormSubmittedTimestamp.Equals(ph.Key, StringComparison.Ordinal))
                {
                    // get the submitted timestamp
                    if (data.TryGetSubmitTime(out DateTime submitted))
                    {
                        data.TryGetLanguage(out string languageCode);

                        try
                        {
                            // the timestamp is DateTimeKind.Utc, format the timestamp using
                            // the forms culture or the default culture
                            ph.Value = $"{submitted.ToString(GetCultureInfo(languageCode))} UTC";
                        }
                        catch {}
                    }
                }
                else if(FormSubmittedBy.Equals(ph.Key, StringComparison.Ordinal))
                {
                    // get the submitted by user
                    if (data.TryGetSubmitUser(out string username))
                    {
                        ph.Value = string.IsNullOrWhiteSpace(username) ? AnonymousUsername : username;
                    }
                    else
                    {
                        ph.Value = AnonymousUsername;
                    }
                }
                else if (FormSubmittedFromPage.Equals(ph.Key, StringComparison.Ordinal))
                {
                    // get the form hosted page
                    if (data.TryGetHostedPage(out ContentReference hostedPage))
                    {
                        LoaderOptions loadingOptions;

                        // try to get the language code
                        if (data.TryGetLanguage(out string languageCode))
                        {
                            loadingOptions = new LoaderOptions { LanguageLoaderOption.FallbackWithMaster(GetCultureInfo(languageCode)) };
                        }
                        else
                        {
                            loadingOptions = new LoaderOptions { LanguageLoaderOption.MasterLanguage() };
                        }

                        // try to load the content in the forms language and fallback to masterlanguage
                        if (_contentLoader.Service.TryGet(hostedPage, loadingOptions, out PageData page))
                        {
                            // and then resolve the url using the language the content was actually loaded in
                            var pageUrl = _urlResolver.Service.GetUrl(page);
                            ph.Value = $"{page.Name}, {pageUrl}";
                        }
                    }
                }
            }

            return availablePlaceHolders;
        }

        /// <summary>
        /// Get <see cref="CultureInfo"/> object using the <paramref name="languageCode"/>.
        /// </summary>
        /// <param name="languageCode">Language code (<see cref="CultureInfo.Name"/>).</param>
        /// <returns><see cref="CultureInfo"/> object created using the <paramref name="languageCode"/> or if there was an exception then a CultureInfo object is returned using  the <see cref="Constants.DefaultCulture"/>.</returns>
        private static CultureInfo GetCultureInfo(string languageCode)
        {
            try
            {
                if (!string.IsNullOrEmpty(languageCode))
                {
                    return CultureInfo.GetCultureInfo(languageCode);
                }
            }
            catch {}

            // this is fallback if there was an exception or the languageCode was null
            return CultureInfo.GetCultureInfo(Constants.DefaultCulture);
        }
    }
}