C# Fill and submit form, then go on url

I am currenlty looking for a C# code where I can "virtually" fill a form from url. I visited most of the pages discussing of the subject. I've read plenty of code that seems to work but does not the thing I want.

Once I've filled the form and submitted it, I want to be able to go on the page. Exactly like if a user had filled the form and submitted it himself by clicking on the "Log in" button for example. I'm missing that part of the code... I'm able to fill a form and get response from the page but from that I don't know how to open the default browser and go to the page, and be automatically logged in thanks to the automatic filling.

I don't know if I'm very clear, I hope you understood my request :)