The credentials (username and password) for a JMS endpoint can be set by getting the JMS Conduit, and setting them on the JMSConfiguration object. 

Here is an example:

Client client = ClientProxy.getClient(greeter);
Conduit conduit=client.getConduit();
JMSConduit jmsConduit=(JMSConduit)conduit;
JMSConfiguration jmsConfiguration=jmsConduit.getJmsConfig();
String user= "username";
String password="password";
jmsConfiguration.getJndiConfig().setConnectionUserName(user);
jmsConfiguration.getJndiConfig().setConnectionPassword(password);