How to write Dot Net (C#) Client for accessing
HTTP interface of SMS Gateway?

Kindly refer the below links for .NET TLS support and upgradation:-

• https://docs.microsoft.com/en-us/dotnet/framework/network-programming/tls
  
  https://blogs.perficient.com/2016/04/28/tsl-1-2-and-net-support/
  
  To download the SMSHttpPostClient C# Code, please login to Services Portal , navigate to Integration Section → Push SMS Integration → SMSHttpPostClient C# Code.