How can I add a SOAPResponseAssertion to a TestStep using Java API?

I am able to create the testcase and teststep programmatically using the following.

WsdlProject project = new WsdlProject(projectFilePath);
WsdlTestSuite testSuite = project.addNewTestSuite("TestSuite");
WsdlTestCase tc = testSuite.addNewTestCase("TestCase" + (i+1));
TestStepConfig config = WsdlTestRequestStepFactory.createConfig(operation, "TestStep1");
WsdlTestStep ts = tc.addTestStep(config);

How do I now add a SOAP Response assertion?

Thanks much in advance!