protected String getAuthenticationClassReference(final HttpServletRequest request,
                                                     final HttpServletResponse response) {
        val context = new JEEContext(request, response);
        val acr = oauthRequestParameterResolver.resolveRequestParameter(context, OAuth20Constants.ACR_VALUES)
            .orElse(StringUtils.EMPTY);
        if (StringUtils.isBlank(acr)) {
            val serviceParam = request.getParameter(CasProtocolConstants.PARAMETER_SERVICE);
            if (StringUtils.isNotBlank(serviceParam)) {
                val queryParams = Unchecked.supplier(
                    () -> new URIBuilder(UriComponentsBuilder.fromUriString(serviceParam).toUriString()).getQueryParams()).get();
                val parameter = queryParams
                    .stream()
                    .filter(p -> p.getName().equals(OAuth20Constants.ACR_VALUES))
                    .findFirst();
                if (parameter.isPresent()) {
                    return EncodingUtils.urlDecode(parameter.get().getValue());
                }
            }
        }
        return EncodingUtils.urlDecode(acr);
    }
This is {@link OidcMultifactorAuthenticationTrigger}.

@author Misagh Moayyed
@since 6.0.0
support/cas-server-support-oidc-core-api/src/main/java/org/apereo/cas/oidc/web/flow/OidcMultifactorAuthenticationTrigger.java