REST Service how to accept a Json request

I have a REST Service that I want it to receive a json string request I have set up my URLMap as follows the top URL when populated and requested works fine but I would like my request to be a json string and that is not working I am getting a 500 error am I missing something please advice.

XData UrlMap
{
<Routes>
<Route Url="/:emailAddress/:sendUserEmail/:password" Method="POST" Call="ResetPassword"/>        this works fine
<Route Url="/test" Method="POST" Call="test"/>              I would like this to receive a json formatted string for the above
</Routes>
}