EWS Proxy Modern Auth Sample

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net;
using System.Threading.Tasks;
using System.Net.Http;
using Microsoft.Identity.Client;
using Newtonsoft.Json;
using EWSWSDLoAuthExample.com.office365.outlook;


namespace EWSWSDLoAuthExample
{
	class Program
	{
		static void Main(string[] args)
		{
			OAuthExchangeServiceBinding exchangeServiceBinding = new OAuthExchangeServiceBinding();
			String ClientId = "111111-52b3-4102-aeff-aad2292ab01c";
			String MailboxName = "user@domain.onmicrosoft.com";
			HttpClient httpClient = new HttpClient();
			httpClient.DefaultRequestHeaders.UserAgent.ParseAdd("Mozilla/5.0 (compatible; AcmeInc/1.0)");
			StringContent RealmRequest = new StringContent("{\"username\":\"" + MailboxName + "\"}");
			String DiscoveryURL = "https://login.microsoftonline.com/common/GetCredentialType";
			dynamic RealmDiscover = JsonConvert.DeserializeObject(httpClient.PostAsync(DiscoveryURL, RealmRequest).Result.Content.ReadAsStringAsync().Result);
			if ((Int32)RealmDiscover.EstsProperties.DomainType == 1 || (Int32)RealmDiscover.EstsProperties.DomainType == 2)
			{
				exchangeServiceBinding.Credentials = new NetworkCredential("user1@contoso.com", "password");
				exchangeServiceBinding.RequestServerVersionValue = new RequestServerVersion();
				exchangeServiceBinding.RequestServerVersionValue.Version = ExchangeVersionType.Exchange2010_SP2;
			}
			else
			{
				String AutoDiscoverEndpoint = $"https://outlook.office365.com/autodiscover/autodiscover.json/v1.0/{MailboxName}?Protocol=EWS";
				dynamic JsonResult = JsonConvert.DeserializeObject(httpClient.GetAsync(AutoDiscoverEndpoint).Result.Content.ReadAsStringAsync().Result);
				if (IsPropertyExist(JsonResult, "Url"))
				{
					String AudienceHostName = new Uri(JsonResult.Url.ToString()).Host;
					string scope = "https://" + AudienceHostName + "/EWS.AccessAsUser.All";
					PublicClientApplicationBuilder pcaConfig = PublicClientApplicationBuilder.Create(ClientId).WithAuthority(AadAuthorityAudience.AzureAdMultipleOrgs);
					exchangeServiceBinding.PublicClientApplication = pcaConfig.Build();
					exchangeServiceBinding.oAuthScopes = new[] { scope };
					exchangeServiceBinding.AnchorMailbox = MailboxName;
					var IntToken = exchangeServiceBinding.PublicClientApplication.AcquireTokenInteractive(exchangeServiceBinding.oAuthScopes).ExecuteAsync().Result;
					exchangeServiceBinding.LogonHint = IntToken.Account.Username;
					exchangeServiceBinding.Url = JsonResult.Url.ToString();
					exchangeServiceBinding.RequestServerVersionValue = new RequestServerVersion();
					exchangeServiceBinding.RequestServerVersionValue.Version = ExchangeVersionType.Exchange2016;
				}
			}

			GetMailTipsType GetMailTips = new GetMailTipsType();
			GetMailTips.MailTipsRequested = new MailTipTypes();
			GetMailTips.MailTipsRequested = MailTipTypes.OutOfOfficeMessage;
			GetMailTips.Recipients = new EmailAddressType[1];
			GetMailTips.Recipients[0] = new EmailAddressType();
			GetMailTips.Recipients[0].EmailAddress = MailboxName;
			GetMailTips.SendingAs = new EmailAddressType();
			GetMailTips.SendingAs.EmailAddress = MailboxName;
			GetMailTipsResponseMessageType GetMailTipsResponse = null;
			try
			{
				GetMailTipsResponse = exchangeServiceBinding.GetMailTips(GetMailTips);
				Console.WriteLine("OOFTip" + GetMailTipsResponse.ResponseMessages[0].MailTips.OutOfOffice.ReplyBody.Message);
			}
			catch (Exception Exception)
			{
				//To Do
			}



		}
		public static bool IsPropertyExist(dynamic settings, string name)
		{
			if (settings is Newtonsoft.Json.Linq.JObject)
				return ((Newtonsoft.Json.Linq.JObject)settings).ContainsKey(name);

			return settings.GetType().GetProperty(name) != null;
		}
	}

	class OAuthExchangeServiceBinding : ExchangeServiceBinding
	{
		public String AnchorMailbox { get; set; }
		public IPublicClientApplication PublicClientApplication { get; set; }
		public String[] oAuthScopes { get; set; }
		public String LogonHint { get; set; }

		protected override System.Net.WebResponse GetWebResponse(System.Net.WebRequest request)
		{

			if (!String.IsNullOrEmpty(AnchorMailbox))
			{
				request.Headers.Add("X-AnchorMailbox", AnchorMailbox);
			}
			if (PublicClientApplication != null)
			{
				Task<AuthenticationResult> authtask = null;
				request.Headers.Remove("Authorization");
				try
				{
					authtask = Task.Run<AuthenticationResult>(async () => await PublicClientApplication.AcquireTokenSilent(oAuthScopes, LogonHint).ExecuteAsync());
				}
				catch (MsalUiRequiredException)
				{
					authtask = Task.Run<AuthenticationResult>(async () => await PublicClientApplication.AcquireTokenInteractive(oAuthScopes).ExecuteAsync());
				}
				request.Headers.Add("Authorization", "Bearer " + authtask.Result.AccessToken);
			}
			System.Net.HttpWebResponse
			response = (System.Net.HttpWebResponse)base.GetWebResponse(request);
			return response;
		}
	}
}

We have EWS proxy in our code and trying to use your suggestion to use modern authentication.
When I am trying to use a token generated from a registered app. However, I get the exception,"ExchangeImpersonation SOAP header must be present for this type of OAuth token." in GetWebResponse.
Can you please suggest what I should do?