I am working on a Windows app with C# as a programming langugage.
Requirement is
To answer your question we should first make some things clear:
(1) You do not login to PuTTY. You use PuTTY to establish a connection to a remote server and to start a shell. Therefore you login to the server hostname
(2) PuTTY is a program to provide an interactive shell to the user. It is not intended to be used as an programming interface
What you look for is a SSH client library such as https://sshnet.codeplex.com/ (never used it, stated here as an example). Such libraries allow you to send commands using .NET using a predefined API