I think that this information is a custom properties in the TestStep.
When you create a testStep with authentication the soapUI generate a singular custom properties(Domain,Username,Password)

The groovy script for to change this properties will be:
//Print Values Properties
log.info testRunner.testCase.getTestStepByName('NAME_TestStepRequest').getPropertyList().name;
log.info testRunner.testCase.getTestStepByName('NAME_TestStepRequest').getPropertyList().value;
log.info testRunner.testCase.getTestStepByName('NAME_TestStepRequest').getPropertyValue("Username");
log.info testRunner.testCase.getTestStepByName('NAME_TestStepRequest').getPropertyValue("Password");
log.info testRunner.testCase.getTestStepByName('NAME_TestStepRequest').getPropertyValue("Domain");
// SET Properties
testRunner.testCase.getTestStepByName('NAME_TestStepRequest').setPropertyValue("Username","B");
testRunner.testCase.getTestStepByName('NAME_TestStepRequest').setPropertyValue("Password","B");
testRunner.testCase.getTestStepByName('NAME_TestStepRequest').setPropertyValue("Domain","B");
// Get new Values
log.info testRunner.testCase.getTestStepByName('NAME_TestStepRequest').getPropertyValue("Username");
log.info testRunner.testCase.getTestStepByName('NAME_TestStepRequest').getPropertyValue("Password");
log.info testRunner.testCase.getTestStepByName('NAME_TestStepRequest').getPropertyValue("Domain");