Custom SOAP headers can be added to a request by using the PayloadFactory Mediator in the proxy service as shown in the example below.
<?xml version="1.0" encoding="UTF-8"?>
<definitions xmlns="http://ws.apache.org/ns/synapse">
<proxy name="StockQuoteProxy"
transports="https http"
startOnLoad="true"
trace="disable">
<description/>
<target>
<endpoint>
<address uri="http://localhost:9001/services/SimpleStockQuoteService"/>
</endpoint>
<inSequence>
<log level="full"/>
<payloadFactory media-type="xml">
<format>
<soapenv:Envelope xmlns:soapenv="http://www.w3.org/2003/05/soap-envelope"
xmlns:xsd="http://services.samples/xsd"
xmlns:ser="http://services.samples">
<soapenv:Header>
<ser:authenticationRequest>
<userName xmlns="">$1</userName>
<password xmlns="">$2</password>
</ser:authenticationRequest>
</soapenv:Header>
<soapenv:Body>
<ser:getQuote>
<ser:request>
<xsd:symbol>$3</xsd:symbol>
</ser:request>
</ser:getQuote>
</soapenv:Body>
</soapenv:Envelope>
</format>
<args>
<arg value="punnadi"/>
<arg value="password"/>
<arg value="hello"/>
</args>
</payloadFactory>
</inSequence>
<outSequence>
<send/>
</outSequence>
</target>
<publishWSDL uri="file:repository/samples/resources/proxy/sample_proxy_1.wsdl"/>
</proxy>
<sequence name="fault">
<log level="full">
<property name="MESSAGE" value="Executing default "fault" sequence"/>
<property name="ERROR_CODE" expression="get-property('ERROR_CODE')"/>
<property name="ERROR_MESSAGE" expression="get-property('ERROR_MESSAGE')"/>
</log>
<drop/>
</sequence>
<sequence name="main">
<log/>
<drop/>
</sequence>
</definitions>