Все гораздо проще:
string content =
"<HTML><HEAD></HEAD><body onLoad='submitForm()'>"+
"</FORM><form id=YourForm action=\"you_url \" METHOD=\"POST\" >"+
"<input type= \"hidden\" name=\"username\" value=\"username\" >"+
"<input type=\"hidden\" name=\"password\" value=\"password\" ></form>"+
"<script>document.forms[0].submit();</script></Body></HTML>";
Response.Clear();
Response.Write(content);
И всего делов
