External POST form submit to ZK's URL without explicit zul page

I need to submit a POST form to a zk application but I cannot retrieve the parameters because the request is always empty. My POST form looks like:

and my ZK app has an Initiator that retrieves form's parameters. I spent 3 days banging my head against this problem, trying to understand if I submitted a wrong form, or whether it was the Initiator class or even Apache Shiro that could clean up the request or anything else. Today I found out that if I submit a POST form to a url that includes the zul page name (e.g http://localhost/myapp/index.zul) it works, but if I don't specify the zul page, it doesn't. I did a very simple test, with just a index.zul and a ViewModel: no Initiator, no Shiro no nothing and I could replicate that behaviour. Is there any explanation for this or is there any solution that allows me to not specify the zul page? Thank you