Forcing TLS 1.2

Use the following line of code to force usage of TLS 1.2.

System.Net.ServicePointManager.SecurityProtocol = System.Net.SecurityProtocolType.Tls12;

Note that this is not the recommended way to do this. The recommended method is to use the v4.7 of the .Net Framework.
See: Transport Layer Security (TLS) best practices with the .NET Framework