I Prefer you to use
he.InnerText="samplePassword";
instead for
he.SetAttribute("value", "text");
; SO the Event should be like the following:
private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
{
foreach (HtmlElement he in webBrowser1.Document.All.GetElementsByName("prijava[password]"))
{
he.InnerText="samplePassword"
}
}