Manipulate page with WebBrowser

The method WebBrowser.Navigate loads the page asynchronously, for you to manipulate the document you must wait until the document has been loaded.

You can do this in two ways:

1. scheduling the event WebBrowser.DocumentCompleted (solution cited in this post )
2. put the code below to ensure that the code does not advance until the page has been loaded (this must be done after the Navigate method and after the call form.InvokeMember("submit");) (solution cited in this post):

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

I made an example working with the code below (it loads the Stackoverflow page and does a search for a keyword). I created a simple Windows application with a form, a WebBrowser component (which I named oWebBrowser just like you) and a button. All the code executes on the click of the button, 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 believe you must be having trouble selecting the control correctly. I applied the same solution idea above, on the login page you mentioned and actually had to take some care to find the controls and set the values.

Tips for you: if I searched for an element with the name "User", it would not return the login input. In this case then I first searched the form by Id oWebBrowser.Document.GetElementById("frmLogin"). Then I searched for the user name control inside the found Form form.GetElementsByTagName("input").GetElementsByName("user")[0]. The same idea was made for the password.

Applying the code below, it is made the login attempt, but obviously, I do not have a valid user and password. See, please, if you can solve 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();
            }