Hi Duy,
If I understood you correctly you want to send a POST request, receive some token back, process it and send a GET request.
You can do it with normal jQuery/javascript AJAX requests, it doesn't matter if you are in Portal or tomcat etc.
in jQuery just use:
$.post( { url: "your url" } );
$.get( { url: "your url" } );
read about the methods in jQuery.com
in javascript use the XMLHTTPRequest:
xmlhttp=new XMLHttpRequest();
xmlhttp.open("GET","books.xml",false);
xmlhttp.send();
BR,
Saar