There are several of us who have been trying to do something simple like this. First POST doesn't work so good from the xml, get works fine. There is a work around It's better to use a .
js file and create a form and use javascript to send the post. I have some code at work that can get the username for you. I'll try to post it tomorrow.
If you do a search for post or qpmp.com you should find the tread talking about post.
Here it is
Post and params
Also I think it's
<param name="USER">{obj.user}</param> - or something close.
If you look at the example in the developers guide (see the wiki), it shows the format.