This looks to be the easiest way to do it:

provideAppInitializer(async () => {
  const keycloakConfig = await inject(ConfigService).getConfig();

  if (keycloakConfig) {
    return provideKeycloak({
      config: {
        url: keycloakConfig.url,
        realm: keycloakConfig.realm,
        clientId: keycloakConfig.clientId
      }
    });
  } else {
    return null;
  }
})