Call HTTP POST method with parameters in hierarchical request structure

Hi,

I have a requirement to pass request to HTTP web service as follows.

{ XYZ:

{ 'Par1': 'Val1',

'Par2': 'Val2'

}

}

i am able to use method set form fields to set fields Par1 and Par2 in request structure but getting error as "Failed to transform from "json" to "java.util.HashMap". (org.mule.api.transformer.TransformerMessagingException)."

Can you please suggest how to pass XYZ to request?

thank you.