@Override
    protected Event doExecuteInternal(final RequestContext requestContext) {
        val webAuthn = casProperties.getAuthn().getMfa().getWebAuthn().getCore();
        val authn = WebUtils.getAuthentication(requestContext);
        val principal = resolvePrincipal(authn.getPrincipal(), requestContext);
        val attributes = principal.getAttributes();

        LOGGER.debug("Starting registration sequence for [{}]", principal);
        val flowScope = requestContext.getFlowScope();
        if (attributes.containsKey(webAuthn.getDisplayNameAttribute())) {
            CollectionUtils.firstElement(attributes.get(webAuthn.getDisplayNameAttribute()))
                .ifPresent(value -> flowScope.put("displayName", value));
        } else {
            flowScope.put("displayName", principal.getId());
        }
        flowScope.put(FLOW_SCOPE_WEB_AUTHN_APPLICATION_ID, webAuthn.getApplicationId());
        return null;
    }
Attribute name that points to the web application id put into the webflow.
support/cas-server-support-webauthn-core-webflow/src/main/java/org/apereo/cas/webauthn/web/flow/WebAuthnStartRegistrationAction.java