import javax.xml.soap.*;
import javax.xml.transform.*;

public class Test {
    public static void main(String[] args) throws Exception {
        MessageFactory factory = MessageFactory.newInstance();
        SOAPMessage message = factory.createMessage();
        SOAPElement first = message.getSOAPBody().addChildElement("ns","","http://test.ru");
        first.addChildElement("String_1","","").setValue("username");


        TransformerFactory tff = TransformerFactory.newInstance();
        Transformer tf = tff.newTransformer();
        tf.setOutputProperty(OutputKeys.INDENT, "yes");
        tf.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "2");

        Source sc = message.getSOAPPart().getContent();
        StreamResult result = new StreamResult(System.out);
        tf.transform(sc, result);
    }
}

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
  <SOAP-ENV:Header/>
  <SOAP-ENV:Body>
    <ns xmlns="http://test.ru">
      <String_1 xmlns="">username</String_1>
    </ns>
  </SOAP-ENV:Body>
</SOAP-ENV:Envelope>