I have used Keygens and when I click generate it places the key into the boxes, I have made my own program that holds my keys and I would like to know how do I press a button and make that key go into the boxes? What would the code look like, I'm a beginner at C# so I don't know much.
I am not sure if C# is the best choice but the process is you should hook the WndProc and add to the text and then pass the event to the necessary control.
Get a book on C# and start on learning the language because what you are looking for needs some knowledge of C# as well as windows programming.