Issues with TLS and adding subscribers

[robertjf]

We have implemented adding Subscribers to a form using the latest campaignmonitor-api Nuget package (v. 6.0.2) and have been having trouble getting the connection to work.

Environment:

Umbraco 13 (dotnet 8)
TLS 1.2 and 1.3 supported

When we first start the site, the system uses TLS 1.2 (logged as "SystemDefault") which works, however if we wait a few minutes and try again, it uses TLS 1.3 which fails with the following error:

System.Net.WebException: The SSL connection could not be established, see inner exception.
---> System.Net.Http.HttpRequestException: The SSL connection could not be established, see inner exception.
---> System.Security.Authentication.AuthenticationException: Authentication failed, see inner exception.
---> System.ComponentModel.Win32Exception (0x80090331): The client and server cannot communicate, because they do not possess a common algorithm.

I don't know what I can do to stop this behaviour, and I don't think it's a good idea to force it to just Tls1.2, is there a way Campaign Monitor can support TLS 1.3 on the create-send api?

Our code for testing is as follows:

            AuthenticationDetails auth = new ApiKeyAuthenticationDetails(ApiKey);
            Subscriber subscriber = new(auth, ListId);
            logger.LogDebug("SecurityProtocol: {protocol}", ServicePointManager.SecurityProtocol);
            
            var result = subscriber.Add(email, name, customFields, true, track, mobileNumber: mobile, sendSms);