How to publish message to SNS in Windows Forms (NET6) app?

I have created SNS topic FIFO, my c# app queried all topics and see all of them including mine.

Then I'd like to publish message to the topic. I use this code, it works in NET6 app but it hangs on PublishAsync when I use it in Windows Forms (NET6)

 public async Task<string> PublishMessageHandler103(string messageText, string msgGroup)
 {
     try
     {
         string topicArn = topicarn;
         var awsCredentials = new Amazon.Runtime.BasicAWSCredentials(accKey, secKey);
         var client = new Amazon.SimpleNotificationService.AmazonSimpleNotificationSer‌​viceClient(awsCreden‌​tials, regionEndpoint);
         var request = new PublishRequest
         {
             TopicArn = topicArn,
             Message = messageText,
             MessageGroupId = msgGroup
         };
         var response = await client.PublishAsync(request);// <----Hangs
         Console.WriteLine($"Successfully published message ID: {response.MessageId}");
         return $"Message Published: {msg}";
     }
     catch (Exception ex)
     {
         Console.WriteLine("\n\n{0}", ex.Message);
     }
     return "Hmmm";
 }