XML : Constructing FormParam request in WSO2

Constructing FormParam request in WSO2 to make a request to a REST Service which will accept FormParam with some specified key. Client will just make a request with XML in form of string to esb to invoke that service which is configured in wso2 esb. We need to take that XML and construct form inside esb and add that XML as value with some specified key which will be accepted by REST EndPoint.