[keycloak-user] keycloak client protocol mapper (script mapper) to add request header into token

When I'm requesting a token from keycloak I want a specific header value (or extra form data) that was supplied in the request to be put in the JWT payload of the generated token. I've tried using a Script Mapper to get access to header values but I can't see how to get access to header values or data in the form data sent in any of the available script variables: user, realm, userSession, keyclockSession. I was hoping I could use a script something like this: httpRequest.getHeader('X-CID');