Read the Request.Form NameValueCollection and process your logic accordingly:
NameValueCollection nvc = Request.Form;
string userName, password;
if (!string.IsNullOrEmpty(nvc["txtUserName"]))
{
userName = nvc["txtUserName"];
}
if (!string.IsNullOrEmpty(nvc["txtPassword"]))
{
password = nvc["txtPassword"];
}
//Process login
CheckLogin(userName, password);
... where "txtUserName" and "txtPassword" are the Names of the controls on the posting page.