@davidlist There’s no way of running any code if you’re using the native signup box, you need to create your own login/signup page, take our website’s login page as an example, and since you’re familiar with the backend, you should NOT use the datasets as they’re not as secure as running your code on the backend, datasets are frontend applications after all.
You can follow my example if you just want to create a blank entry for the user in the database, and you’ll still be able to use the datasets, or even better, you can go all backend, and have a function that retrieves users’ entries, and update them if necessary, this is the pro way to do it.