Integration with Paygate

The PayGate API only has a PHP example, and the example that I could find uses a HTML file to submit. I require a solution whereby the integration into the PayGate API is executed from within an ASPX file, initiated with a <asp:button>'s OnClick event, with the callback and notification implemented. The code executed needs to be contained in the code behind file, or being fully contained inside the ASPX file with scripting. I prefer the implementation with C# in code behind. Details of the project output can be negotiated, along with the final price. The reason I require this help is mainly because I don't know how to handle the API calls into PayGate with their requirements of Hidden Form fields, and how to handle that with the HttpClient construct. I don't require any UI capability, only the code to call the API and handle the Notification callback as well as the Response.