In order to use TLS 1.2 in .NET 3.5, we need to use third party library because it is not supported by .NET API.
Microsoft enables TLS 1.2 started from .NET 4.5.
So, the solution is to use the library below.
They provide support for .NET 4.0. However, to be able to use it in .NET 3.5 I need to recompile the project for appropariate environment.