Basically, you use send to pass a GET line.

Code:
var parameters = "limit=value&otheritem=itsvalue";

ajaxRequest.open('POST', url, true);
ajaxRequest.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
ajaxRequest.setRequestHeader("Content-length", parameters.length);
ajaxRequest.setRequestHeader("Connection", "close");
ajaxRequest.send(parameters);