I have a Web Service that is expecting it as a part of SOAP header. How do I send it?

<soapenv:Header>

<wsse:Security

xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-

secext-1.0.xsd">

<wsse:UsernameToken>

<wsse:Username>username</wsse:Username>

<wsse:Password>password</wsse:Password>

</wsse:UsernameToken>

</wsse:Security>

</soapenv:Header>
<soapenv:Body>
<actual callout method here>
</soapenv:Body>

The only documentation I see is for inputHttpHeaders_x which can send http header and can send username password as part of 'Authorization' Header.