bootstrapper.cs

private static void Bootstrapper_Bootstrapped(object sender, EventArgs e)
		{
	

			//Register pre render event to add meta data for pages
			EventHub.Subscribe<IPagePreRenderCompleteEvent>(PageMetaDataHelper.OnPagePreRenderCompleteEventHandler);

		
		}

PageMetaDataHelper.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using Telerik.Sitefinity.Services;
using Telerik.Sitefinity.Web.Events;
using Telerik.Sitefinity.Libraries.Model;
using System.Web.UI;
using System.Web.UI.HtmlControls;

namespace CLane.Extensions.Pages
{
	public static class PageMetaDataHelper
	{
		/// <summary>
		/// Page prerender event to handle placing the meta tags
		/// </summary>
		/// <param name="evt">Prerender event</param>
		public static 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()));
			}
		}

		/// <summary>
		/// Try to add meta field to head
		/// </summary>
		/// <param name="evt">Prerender event</param>
		/// <param name="metaFieldIdentifier">Meta field identifier</param>
		/// <param name="pageFieldIdentifier">Sitefinity page field/property name</param>
		private static 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));
			}
		}

		/// <summary>
		/// Does the tag already exist in the page header
		/// </summary>
		/// <param name="page">Page being rendered</param>
		/// <param name="tagPropertyTitle">Meta tag property title</param>
		/// <returns>If the tag already exists in the page header</returns>
		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));
		}

		/// <summary>
		/// Build out meta tag for insertion
		/// </summary>
		/// <param name="property">Meta property name</param>
		/// <param name="content">Value of meta field</param>
		/// <returns>HtmlMeta tag element</returns>
		private static HtmlMeta BuildOGMetaTag(string property, string content)
		{
			var meta = new HtmlMeta() { Content = content };
			meta.Attributes.Add("property", property);
			return meta;
		}
	}
}