Render OG Fields to Page Head

Global.asax

using SitefinityWebApp.Helpers;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web.Http;
using System.Web.Routing;
using Telerik.Sitefinity.Abstractions;
using Telerik.Sitefinity.Data;
using Telerik.Sitefinity.Frontend;
using Telerik.Sitefinity.Frontend.Navigation.Mvc.Models;
using Telerik.Sitefinity.Services;
using Telerik.Sitefinity.Web.Events;
using Telerik.Sitefinity.Model;
using Telerik.Sitefinity;
using Telerik.Sitefinity.Libraries.Model;
using System.Web.UI.HtmlControls;
using System.Web.UI;

namespace SitefinityWebApp
{
	public class Global : System.Web.HttpApplication
	{

		protected void Application_Start(object sender, EventArgs e)
		{
			SystemManager.ApplicationStart += SystemManager_ApplicationStart;
		}

		private void SystemManager_ApplicationStart(object sender, EventArgs e)
		{
			EventHub.Subscribe<IPagePreRenderCompleteEvent>(this.OnPagePreRenderCompleteEventHandler);
		}

		private void OnPagePreRenderCompleteEventHandler(IPagePreRenderCompleteEvent evt)
		{
			TryAddMetaField(evt, "og:title", "OGTitle");
			TryAddMetaField(evt, "og:description", "OGDescription");
			TryAddMetaField(evt, "og:image", "OGImage");
			string ogSiteName = "og:site_name";
			if (!DoesTagExist(evt.Page, ogSiteName))
			{
				evt.Page.Header.Controls.Add(BuildOGMetaTag(ogSiteName, SystemManager.CurrentContext.CurrentSite.Name));
			}

			string ogUrl = "og:url";
			if (!DoesTagExist(evt.Page, ogUrl))
			{
				evt.Page.Header.Controls.Add(BuildOGMetaTag(ogUrl, SystemManager.CurrentHttpContext.Request.Url.ToString()));
			}
		}

		private void TryAddMetaField(IPagePreRenderCompleteEvent evt, string metaFieldIdentifier, string pageFieldIdentifier)
		{
			var page = evt.Page;
			var pageNode = evt.PageSiteNode;
			var field = pageNode.GetCustomFieldValue(pageFieldIdentifier);
			string fieldValue = string.Empty;
			if (field is string)
			{
				fieldValue = field as string;
			}
			else if (field is Image img)
			{
				if (img != null)
				{
					fieldValue = img.Url;
				}
			}

			if (!fieldValue.IsNullOrWhitespace() && !DoesTagExist(page, metaFieldIdentifier))
				page.Header.Controls.Add(BuildOGMetaTag(metaFieldIdentifier, fieldValue));
		}

		private static bool DoesTagExist(Page page, string tagPropertyTitle)
		{
			//Sitefinity adds it's OG tags via literals, so we check them here
			return page.Header.Controls.OfType<LiteralControl>().Any(control => control.Text.Contains(tagPropertyTitle));
		}

		private HtmlMeta BuildOGMetaTag(string property, string content)
		{
			var meta = new HtmlMeta() { Content = content };
			meta.Attributes.Add("property", property);
			return meta;
		}
	}
}