found it here: https://community.jboss.org/wiki/WebAuthenticationUsingHTTPRequestParameters
I'm using maven so needed to add the following to my dependencies:
| | <dependency> |
| | <groupId>org.jboss.spec.javax.security.jacc</groupId> |
| | <artifactId>jboss-jacc-api_1.4_spec</artifactId> |
| | </dependency> |
(I've not got a version number in there because I imported the jboss bom which specifies 1.0.0.Final for me)