When a user signs up I need to store a unique record identifier in my own custom-database. How would I go about inserting the data into my own database? Is there a way to link the Auth0 login ID with my own database record?
For example, if your integration only uses database and passwordless connections a possible choice to execute custom logic after a user signup would be the (https://auth0.com/docs/hooks/concepts/post-user-registration-extensibility-point).
If you need that logic to be executed for several different connection types you can consider using a rule (https://auth0.com/docs/rules) with additional logic to ensure that it only runs the identifier related logic once per user. This would be required because rules execute multiple times (for example, they run at each user login).
Given both hooks and rules allow you run a wide variety of Node.js modules and logic, including performing external network request you could use them to store something on your own system the first time a users completes a registration through Auth0.