How to fill and submit a web form using C# code?

Question

I am doing a project in which I have to make a windows application that can Take a URL in textbox from user. Now when the user press the Proceed button, the application should open that URl in a webbrowser control and fill the form on that page containing userID & password textboxes and submit it via the login button on that web page. Now my application should show the next page in that webbrowser control to the user.

I can open the url in the application's webbrowser control through my C# Code, but I can't figure it out that how to find the userID & pasword textboxes on that web page that is currently opened in the webbrowser control of my application, how to fill them, how to find the login button & how to click it through my C# Code.