I got around this issue by passing the username and password in the url:
example:
def builder = new RestRequestObjectBuilder()
‘Create a new POST object using builder’
def requestObject = builder
.withRestRequestMethod(“POST”)
.withRestUrl(‘https://DOMAIN\USERNAME:PASSWORD@yoururl.com/something’)
.withHttpHeaders([new TestObjectProperty(“Content-Type”, ConditionType.EQUALS, “application/json”),])
.build()
ResponseObject Response = WS.sendRequest(requestObject)
Hope this helps!