Add SoapHeader to org.springframework.ws.WebServiceMessage

You can do as below:

public class SoapRequestHeaderModifier implements WebServiceMessageCallback {
 private final String userName = "user";
 private final String passWd = "passwd";

 @Override
 public void doWithMessage(WebServiceMessage message) throws IOException, TransformerException {
  if (message instanceof SaajSoapMessage) {
   SaajSoapMessage soapMessage = (SaajSoapMessage) message;
   MimeHeaders mimeHeader = soapMessage.getSaajMessage().getMimeHeaders();
   mimeHeader.setHeader("Authorization", getB64Auth(userName, passWd));
  }
 }

 private String getB64Auth(String login, String pass) {
  String source = login + ":" + pass;
  String retunVal = "Basic " + Base64.getUrlEncoder().encodeToString(source.getBytes());
  return retunVal;
 }
}

Then

Object response = getWebServiceTemplate().marshalSendAndReceive(request, new SoapRequestHeaderModifier());

Basically, you need to use a WebServiceMessageCallback in your client to modify the message after its creation but before it is sent. To rest of the code has been described pretty accurately by @skaffman so the whole stuff might look like this:

public void marshalWithSoapActionHeader(MyObject o) {

    webServiceTemplate.marshalSendAndReceive(o, new WebServiceMessageCallback() {

        public void doWithMessage(WebServiceMessage message) {
            try {
                SoapMessage soapMessage = (SoapMessage)message;
                SoapHeader header = soapMessage.getSoapHeader();
                StringSource headerSource = new StringSource("<credentials xmlns=\"http://example.com/auth\">\n +
                        <username>"+username+"</username>\n +
                        <password>"+password"+</password>\n +
                        </credentials>");
                Transformer transformer = TransformerFactory.newInstance().newTransformer();
                transformer.transform(headerSource, header.getResult());
            } catch (Exception e) {
                // exception handling
            }
        }
    });
}

Personally, I find that Spring-WS sucks hard for such a basic need, they should fix SWS-479.