you can set in proxy as below.
<AssignMessage continueOnError="false" enabled="true" name="set-1">
<Set>
<FormParams>
<FormParam name="myparam">{request.header.myparam}</FormParam>
</FormParams>
<Headers>
<Header name="user-agent">{request.header.user-agent}</Header>
</Headers>
<QueryParams>
<QueryParam name="name">{request.header.name}</QueryParam>
<QueryParam name="address">{request.header.address}</QueryParam>
</QueryParams>
<!-- <Verb>GET</Verb> -->
<Payload contentType="text/plain">42</Payload>
<Path/>
<ReasonPhrase>Bad request</ReasonPhrase>
<StatusCode>400</StatusCode>
<Verb>POST</Verb>
<Verb>{my_variable}</Verb>
<Version>1.1</Version>
</Set>
<AssignTo createNew="false" transport="http" type="response"/>
</AssignMessage>