C# script that detect already opened website and click buttons and inserta data

I would like to write simple scripts which after I have already opened site ( I dont wanna script to open it) press two buttons and insert data in comment section after pressing f.ex. 'g' button. I am completly new in that kind of programming so any help will be nice( also link to good tutorials).

 webBrowser1.Document.GetElementById("User").SetAttribute("value", textBox1.Text);
        webBrowser1.Document.GetElementById("but").InvokeMember("click");

I am aware of those 2 functions i will use but how to instantiate them on already opened page by pressing a button? (If thats important deafult used browser is opera).