title: Message: Subscription validation request failed. Notification endpoint must respond with 200 OK to validation request
body: <p>i'm getting this error this is my code</p>
  <pre><code>public static async Task&lt;ActionResult&lt;string&gt;&gt; Get([HttpTrigger(AuthorizationLevel.Anonymous, &quot;get&quot;, Route = null)] HttpRequestMessage req)        {
          var graphServiceClient = GetGraphClient();
  
          var sub = new Microsoft.Graph.Subscription();
          sub.ChangeType = &quot;updated&quot;;
          sub.NotificationUrl = config.ngrok+&quot;/api/notifications&quot;;
          sub.Resource = &quot;/users&quot;;
          sub.ExpirationDateTime = DateTime.UtcNow.AddMinutes(5);
          sub.ClientState = &quot;SecretClientState&quot;;
  
          var newSubscription = await graphServiceClient
            .Subscriptions
            .Request()
            .AddAsync(sub);
          return $&quot;Subscribed. Id: {newSubscription.Id}, Expiration: {newSubscription.ExpirationDateTime}&quot;;
      }
  </code></pre>
  <p>when subscribing i get the error, it's because of the NotificationUrl? i'm very new at this:/</p>