If you are not familiar with SOAP, check out SOAP in WebSphere - Getting Started.

Recall that the SOAP header is optional and the SOAP body is required. The mustUnderstand attribulte tells the receiving system that the SOAP headed is required (must be consumed). When mustUnderstand is 1 (true), the headed must be consumed. When mustUnderstand is 0 (false), the header is optional.

<?xml version='1.0' Encoding='UTF-8' ?>
<env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope"> 
 <env:Header>
     <wss:UsernameToken>                                              
        <wss:Username>username</wss:Username>                        
        <wss:Password>password</wss:Password>                        
     </wss:UsernameToken>                                             
   </wss:Security> 
 </env:Header>
 <env:Body>
   ...
 </env:Body>
</env:Envelope>