interface

...

type
  TWebForm = class(TForm)
    ...
  private
    DocumentLoaded: boolean;
    ...
  end;

implementation

...

procedure TWebForm.WebBrowserBeforeNavigate2(Sender: TObject;
  const pDisp: IDispatch; var URL, Flags, TargetFrameName, PostData,
  Headers: OleVariant; var Cancel: WordBool);
begin
  if pDisp = WebBrowser.DefaultInterface then
    DocumentLoaded := false;
end;

procedure TDemoForm.WebBrowserDocumentComplete(Sender: TObject;
  const pDisp: IDispatch; var URL: OleVariant);
begin
  if pDisp = WebBrowser.DefaultInterface then
    DocumentLoaded := true;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  WebBrowser.Navigate('http://mail.yahoo.de');
  repeat
    Application.ProcessMessages;
  until DocumentLoaded;

  WebBrowser.OleObject.document.forms.item(0).elements.item('login').value:='******';
  WebBrowser.OleObject.document.forms.item(0).elements.item('passwd').value:='*****';
  WebBrowser.OleObject.document.forms.item(0).submit;
  repeat
    Application.ProcessMessages;
  until DocumentLoaded;

  ClickLink(WebBrowser, 'Posteingang');
end;