WSO2ESB: How to add <?xml into the message sent to POX endpoint - wso2-esb

Question

I'm using wso2ei 6.0.0 and I have a simple task - send specific xml message via POST to the endpoint.
I have API configured like this:
<api xmlns="http://ws.apache.org/ns/synapse" name="glisTest" context="/glisTest">
<resource methods="GET" uri-template="/{sampleid}">
<inSequence>
<property name="PRESERVE_WS_ADDRESSING" value="false"/>
<payloadFactory media-type="xml">
<format>
<register />
</format>
</payloadFactory>
<log level="full"/>
<property name="messageType" value="application/xml" scope="axis2" type="STRING"/>
<property name="HTTP_METHOD" value="post" scope="axis2" type="STRING"/>
<send>
<endpoint>
<address uri="https://localhost/manager" encoding="UTF-8" format="pox"/>
</endpoint>
</send>
</inSequence>
<outSequence>
<log level="full"/>
<send/>
</outSequence>
</resource>
</api>
But service is expected
<?xml version="1.0" encoding="UTF-8" ?>
to be inserted at the beginning and wso2 do not do it (I just added debug logging for org.apache.synapse.transport.http.wire and I see it simple send
<register/>
As result, it does not recognize message as an xml and returm me error.
Question - how to tell synapse to add this special xml header to the message?