code: public String getCallbackUrl(String challenge) {
          return "http://"
                  + Model.getSingleton().getOptionsParam().getProxyParam().getProxyIp() + ":"
                  + Model.getSingleton().getOptionsParam().getProxyParam().getProxyPort() + "/"
                  + getPrefix() + "/"
                  + challenge;
      }
docs: Gets the ZAP API URL to a challenge endpoint.
  
  @param challenge the last segment of the path for the challenge endpoint
  @return a ZAP API URL to access the the challenge endpoint