how to validate SAP portal username and password with javascript from adobe form

In my Adobe interactive form, on the click of a button, a pop up comes for User ID and Password inputs.

I am trying to send this user id and password to a HTTPS url of SAP Portal and get a response back (return code 200, I think).

How to achieve this?

I am using the following code but it is not returning any thing

var statustext;
var xmlhttp;
xmlhttp = new XMLHttpRequest();
xmlhttp.open("GET",cPortalUrl,true);
xmlhttp.send();

statustext = xmlhttp.responseTEXT;