Manipulate page with WebBrowser

The method WebBrowser.Navigate load the page asynchronously, so you manipulate the document should wait for the document to be loaded.

You can do this in two ways:

while (oWebBrowser.ReadyState != WebBrowserReadyState.Complete) { Application.DoEvents(); }

I made an example working with the code below (It loads the Stackoverflow page and searches for a keyword). I created a simple Windows application with a form, a WebBrowser component (which named the WebBrowser just like you) and a button. All the code executes in the button click, which is below:

        private void button1_Click(object sender, EventArgs e)
        {
            //Resposta a pergunta http://pt.stackoverflow.com/questions/109853/manipular-p%C3%A1gina-com-webbrowser
            oWebBrowser.ScriptErrorsSuppressed = true;
            oWebBrowser.Navigate("https://stackoverflow.com/");

            //https://stackoverflow.com/questions/583897/c-sharp-how-to-wait-for-a-webpage-to-finish-loading-before-continuing
            while (oWebBrowser.ReadyState != WebBrowserReadyState.Complete)
            {
                Application.DoEvents();
            }

            //https://stackoverflow.com/questions/9925022/webbrowser-document-is-always-null
            MessageBox.Show(oWebBrowser.Document.Url.AbsoluteUri);

            HtmlElement questionInput = oWebBrowser.Document.GetElementById("q");
            if (questionInput != null)
            {
                questionInput.SetAttribute("value", "WebBrowser");
                MessageBox.Show("encontrou o campo valor");
            }

            HtmlElement questionForm = oWebBrowser.Document.GetElementById("search");
            if (questionInput != null)
            {
                questionForm.InvokeMember("submit");
                MessageBox.Show("Fez submit");
            }
            while (oWebBrowser.ReadyState != WebBrowserReadyState.Complete)
            {
                Application.DoEvents();
            }    
            MessageBox.Show(oWebBrowser.Document.Url.AbsoluteUri);

            //https://stackoverflow.com/questions/2299273/how-do-i-submit-a-form-inside-a-webbrowser-control
        }

Edit1:

I think you should be having trouble selecting the control correctly. I applied the same idea of solution above on the login page that you mentioned and really had to take some care to find the controls and set the values.

Tips for you: If I were looking for an element with the name "User", I would not return the login input. In that case, then I first searched for the form with the ID oWebBrowser.Document.GetElementById("frmLogin") . Then I searched for the user name control within the form found form.GetElementsByTagName("input").GetElementsByName("user")[0] . The same idea was made for the password.

Applying the code below, the login attempt is made, but obviously, I do not have a valid username and password. Please see if you can solve it with these tips.

            oWebBrowser.ScriptErrorsSuppressed = true;
            oWebBrowser.Navigate("gsmfans.org/index.php?action=login");

            //https://stackoverflow.com/questions/583897/c-sharp-how-to-wait-for-a-webpage-to-finish-loading-before-continuing
            while (oWebBrowser.ReadyState != WebBrowserReadyState.Complete &&
                oWebBrowser.Document == null)
            {
                Application.DoEvents();
            }

            //https://stackoverflow.com/questions/9925022/webbrowser-document-is-always-null
            MessageBox.Show(oWebBrowser.Document.Url.AbsoluteUri);

            HtmlElement form = oWebBrowser.Document.GetElementById("frmLogin");

            HtmlElement userInput = form.GetElementsByTagName("input").GetElementsByName("user")[0];
            if (userInput != null)
            {  //tentativa de setar o subject
                userInput.SetAttribute("value", "TesteUser");
                userInput.ScrollIntoView(true);
                MessageBox.Show("user");
            }

            HtmlElement passwordInput = form.GetElementsByTagName("input").GetElementsByName("passwrd")[0];
            if (passwordInput != null)
            { //tentativa de setar o message
                passwordInput.SetAttribute("value", "TestePass");
                MessageBox.Show("password");
            }

            if (form != null)
            { //tentativa de setar o message
                form.InvokeMember("submit");
                MessageBox.Show("submit");
            }

            while (oWebBrowser.ReadyState != WebBrowserReadyState.Complete &&
                oWebBrowser.ReadyState != WebBrowserReadyState.Complete)
            {
                Application.DoEvents();
            }