FYes you can run your own js code at this stage https://zitadel.com/docs/apis/actions/register-flow#post-creation
FAnd with the http module you can invoke your webhook endpoint https://zitadel.com/docs/apis/actions/modules#http
FAt the moment the action is only implemented on registering user with external idp, on local user is currently in implementation and should be included in the next release