I don't know how much data you need to send upstream, however, here are two options:
1) MAKE AN IMAGE REQUEST THAT CONTAINS ALL PERTINENT DATA IN THE URL:
Parse the data you want to send upstream into query string parameters that get submitted to a special web service that knows how to read and collect this data from the URL. The server response should be empty. NOTE: URLs should not exceed 2000 characters. If you have a large data set, you will want to use option 2.
EXAMPLE:
/* I'd recommend doing the following with jQuery or some other JS framework */
var img = document.createElement('img');
img.src = "http://website2.com/uploadHandler" +
"?data1="+encodeURIComponent(data1) +
"&data2="+encodeURIComponent(data2);
document.getElementsByTagName("body")[0].appendChild( img );
OUTPUT (at end of body tag):
<img src="http://website2.com/uploadHandler?data1=myName&data2=myInformation" />
This will cause a HTTP GET request to be made to your server at the above address. The trick is that you're not actually going to serve an image but rather collect data from the request.
2) FORM POST:
Use javascript to create a form and populate that form with input fields containing the data you wish to upload. You can automatically submit this form using myForm.submit(). Using jQuery this would look something like:
$(document.body).append( $('\
<form name="myform" action="http://website2.com/uploadHandler">\
<input type="text" name="data1" value="myName" />\
<input type="text" name="data2" value="myInformation" />\
</form>') );
document.myform.submit();
Using this technique will cause a new page to load. However, you could use a redirect on the server side to redirect to the original page. Read the following if you choose to redirect: http://www.theserverside.com/news/1365146/Redirect-After-Post