No problem, happy to help where I can!
How exactly are you logging users in? Are you using an OAuth/OIDC flow via an Auth0 SDK? The user_id is available as the sub claim in an ID Token if you are indeed using a flow that results in one. Alternatively, you could get the sub claim from the /userinfo endpoint using the user’s access token. This can all be done outside of an action entirely which might be a more straightforward approach.
From within an action, you might want to look at using an external library such as axios (or any publicly available npm package) to communicate with your service.