Csharp weblogin user

I need your help with this Web C#.

I'm trying to login to : https://idaas.iam.ibm.com/idaas/mtfim/sps/authsvc?PolicyId=urn:ibm:security:authentication:asf:basicldapuser

What i have tried :

private void button3_Click(object sender, EventArgs e)
        {
            webBrowser1.Navigate("https://idaas.iam.ibm.com/idaas/mtfim/sps/authsvc?PolicyId=urn:ibm:security:authentication:asf:basicldapuser");
        }

 private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
        webBrowser1.Document.GetElementById("username").SetAttribute("value", "test@test.com");
        webBrowser1.Document.GetElementById("continue-button").InvokeMember("click");
        }

The problem is : when i invoke "continue-button", the website return that i need to enter an username...