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For your app to negotiate TLS 1.2, the OS and the .NET Framework version both need to support TLS 1.2.

Operating system requirements to support TLS 1.2

To enable or re-enable TLS 1.2 and/or TLS 1.1 on a system that supports them, see Transport Layer Security (TLS) registry settings.

Requirements to support TLS 1.2 with .NET Framework 3.5

This table shows the OS update you'll need to support TLS 1.2 with .NET Framework 3.5. We recommend you apply all OS updates.

Set the DontEnableSystemDefaultTlsVersions AppContext switch to falsethe DontEnableSystemDefaultTlsVersions AppContext switch to false. Whether by default, or by setting them explicitly, the switches should be false if possible.

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If your app targets .NET Framework 4.7 or later versions, this switch defaults to false. That's a secure default that we recommend. If your app runs on .NET Framework 4.7 or later versions, but targets an earlier version, the switch defaults to true. In that case, you should explicitly set it to false.

For .NET Framework 3.5 - 4.5.2

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Set the SchUseStrongCrypto and SystemDefaultTlsVersions registry keys to 1. See Configuring security via the Windows Registry. The .NET Framework version 3.5 supports the SchUseStrongCrypto flag only when an explicit TLS value is passed.

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For .NET Framework 3.5

The .NET framework version 3.5 SP1 and earlier versions did not provide support for applications to use Transport Layer Security (TLS) System Default Versions as a cryptographic protocol. This update enables the use of TLS v1.2 in the .NET Framework 3.5 SP1.

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So in your Application Start entry point (Like the Global.asax, MVC or OWin Startup class) you can add the following line to set TLS 1.2protected void Application_Start(Object sender, EventArgs e)

{

    //Enableprotected void Application_Start(Object sender, EventArgs e)
{
	//The following line enables TLS 1.1 (in case other requests need to support TLS 1.1) and also TLS 1.2 for Quik! 

    	ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12

}

Most of the above explanation was taken from Microsoft documentation, for more details please see https://docs.microsoft.com/en-us/dotnet/framework/network-programming/tls

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