mpfds := TIdMultiPartFormDataStream.Create(Self); 
    mpfds.AddFormField('vb_login_username', 'Вася'); 
    mpfds.AddFormField('vb_login_password', 'Пупкин'); 
   
 IdHTTP.AllowCookies:= True; 
IdHTTP.HandleRedirects:= True; 
idHTTP.Request.UserAgent:= 'Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)'; 
idHTTP.Request.ContentType:='application/x-www-form-urlencoded'; 
idHttp.Post('http://  ....   /login.php?do=login', mpfds);
 mpfds.Free;

<form action="login.php?do=login" method="post" onsubmit="md5hash(vb_login_password, vb_login_md5password, vb_login_md5password_utf, 0)">