Hello,
We have recently added a feature for .Net 3.5 allowing applications to optionally use system-default SSL and TLS versions, including TLS 1.2, rather than the versions of TLS now used by default in .Net 3.5.
To use this feature, you must first install a patch that is specific to each version of Windows, as described in the KB articles below, and then modify the registry as described in each KB article.
KB3154518 - Reliability Rollup HR-1605 - NDP 2.0 SP2 - Win7 SP1/Win 2008 R2 SP1
KB3154519 - Reliability Rollup HR-1605 - NDP 2.0 SP2 - Win8 RTM/Win 2012 RTM
KB3154520 - Reliability Rollup HR-1605 - NDP 2.0 SP2 - Win8.1RTM/Win 2012 R2 RTM
KB3156421 - 1605 HotFix Rollup through Windows Update for Windows 10.
Cristian Pop, Microsoft (MSFT)